St. Thomas Aquinas discussed his view of predestination in a number of places, including his famous Summa Theologica and his Summa Contra Gentiles. Below, I have selected some key portions from both of these works which lay out St. Thomas's views on this subject. The section on predestination from the Summa Theologica can be found here. My selections from the Summa Contra Gentiles can be found here (#159-163 - though this source uses a different translation from the one in my text below). My texts below come from public domain versions of both works, such as can be found here and here.
First, from the Summa Theologica:
It is fitting that God should predestine men. For all things are subject to His providence, as was shown above (I:22:2). Now it belongs to providence to direct things towards their end, as was also said (I:22:1 and I:22:2). The end towards which created things are directed by God is twofold; one which exceeds all proportion and faculty of created nature; and this end is life eternal, that consists in seeing God which is above the nature of every creature, as shown above (I:12:4). The other end, however, is proportionate to created nature, to which end created being can attain according to the power of its nature. Now if a thing cannot attain to something by the power of its nature, it must be directed thereto by another; thus, an arrow is directed by the archer towards a mark. Hence, properly speaking, a rational creature, capable of eternal life, is led towards it, directed, as it were, by God. The reason of that direction pre-exists in God; as in Him is the type of the order of all things towards an end, which we proved above to be providence. Now the type in the mind of the doer of something to be done, is a kind of pre-existence in him of the thing to be done. Hence the type of the aforesaid direction of a rational creature towards the end of life eternal is called predestination. For to destine, is to direct or send. Thus it is clear that predestination, as regards its objects, is a part of providence. (I:23:1)
God does reprobate some. For it was said above (Article 1) that predestination is a part of providence. To providence, however, it belongs to permit certain defects in those things which are subject to providence, as was said above (I:22:2). Thus, as men are ordained to eternal life through the providence of God, it likewise is part of that providence to permit some to fall away from that end; this is called reprobation. Thus, as predestination is a part of providence, in regard to those ordained to eternal salvation, so reprobation is a part of providence in regard to those who turn aside from that end. Hence reprobation implies not only foreknowledge, but also something more, as does providence, as was said above (I:22:1). Therefore, as predestination includes the will to confer grace and glory; so also reprobation includes the will to permit a person to fall into sin, and to impose the punishment of damnation on account of that sin. (I:23:3)
Since predestination includes will, as was said above (Article 4), the reason of predestination must be sought for in the same way as was the reason of the will of God. Now it was shown above (I:19:5), that we cannot assign any cause of the divine will on the part of the act of willing; but a reason can be found on the part of the things willed; inasmuch as God wills one thing on account of something else. Wherefore nobody has been so insane as to say that merit is the cause of divine predestination as regards the act of the predestinator. But this is the question, whether, as regards the effect, predestination has any cause; or what comes to the same thing, whether God pre-ordained that He would give the effect of predestination to anyone on account of any merits.
Accordingly there were some who held that the effect of predestination was pre-ordained for some on account of pre-existing merits in a former life. This was the opinion of Origen, who thought that the souls of men were created in the beginning, and according to the diversity of their works different states were assigned to them in this world when united with the body. The Apostle, however, rebuts this opinion where he says (Romans 9:11-12): "For when they were not yet born, nor had done any good or evil . . . not of works, but of Him that calleth, it was said of her: The elder shall serve the younger."
Others said that pre-existing merits in this life are the reason and cause of the effect of predestination. For the Pelagians taught that the beginning of doing well came from us; and the consummation from God: so that it came about that the effect of predestination was granted to one, and not to another, because the one made a beginning by preparing, whereas the other did not. But against this we have the saying of the Apostle (2 Corinthians 3:5), that "we are not sufficient to think anything of ourselves as of ourselves." Now no principle of action can be imagined previous to the act of thinking. Wherefore it cannot be said that anything begun in us can be the reason of the effect of predestination.
And so others said that merits following the effect of predestination are the reason of predestination; giving us to understand that God gives grace to a person, and pre-ordains that He will give it, because He knows beforehand that He will make good use of that grace, as if a king were to give a horse to a soldier because he knows he will make good use of it. But these seem to have drawn a distinction between that which flows from grace, and that which flows from free will, as if the same thing cannot come from both. It is, however, manifest that what is of grace is the effect of predestination; and this cannot be considered as the reason of predestination, since it is contained in the notion of predestination. Therefore, if anything else in us be the reason of predestination, it will outside the effect of predestination. Now there is no distinction between what flows from free will, and what is of predestination; as there is not distinction between what flows from a secondary cause and from a first cause. For the providence of God produces effects through the operation of secondary causes, as was above shown (I:22:3. Wherefore, that which flows from free-will is also of predestination. We must say, therefore, that the effect of predestination may be considered in a twofold light—in one way, in particular; and thus there is no reason why one effect of predestination should not be the reason or cause of another; a subsequent effect being the reason of a previous effect, as its final cause; and the previous effect being the reason of the subsequent as its meritorious cause, which is reduced to the disposition of the matter. Thus we might say that God pre-ordained to give glory on account of merit, and that He pre-ordained to give grace to merit glory. In another way, the effect of predestination may be considered in general. Thus, it is impossible that the whole of the effect of predestination in general should have any cause as coming from us; because whatsoever is in man disposing him towards salvation, is all included under the effect of predestination; even the preparation for grace. For neither does this happen otherwise than by divine help, according to the prophet Jeremias (Lamentations 5:21): "convert us, O Lord, to Thee, and we shall be converted." Yet predestination has in this way, in regard to its effect, the goodness of God for its reason; towards which the whole effect of predestination is directed as to an end; and from which it proceeds, as from its first moving principle. (I:23:5)
The reason for the predestination of some, and reprobation of others, must be sought for in the goodness of God. Thus He is said to have made all things through His goodness, so that the divine goodness might be represented in things. Now it is necessary that God's goodness, which in itself is one and undivided, should be manifested in many ways in His creation; because creatures in themselves cannot attain to the simplicity of God. Thus it is that for the completion of the universe there are required different grades of being; some of which hold a high and some a low place in the universe. That this multiformity of grades may be preserved in things, God allows some evils, lest many good things should never happen, as was said above (I:22:2). Let us then consider the whole of the human race, as we consider the whole universe. God wills to manifest His goodness in men; in respect to those whom He predestines, by means of His mercy, as sparing them; and in respect of others, whom he reprobates, by means of His justice, in punishing them. This is the reason why God elects some and rejects others. To this the Apostle refers, saying (Romans 9:22-23): "What if God, willing to show His wrath [that is, the vengeance of His justice], and to make His power known, endured [that is, permitted] with much patience vessels of wrath, fitted for destruction; that He might show the riches of His glory on the vessels of mercy, which He hath prepared unto glory" and (2 Timothy 2:20): "But in a great house there are not only vessels of gold and silver; but also of wood and of earth; and some, indeed, unto honor, but some unto dishonor." Yet why He chooses some for glory, and reprobates others, has no reason, except the divine will. Whence Augustine says (Tract. xxvi. in Joan.): "Why He draws one, and another He draws not, seek not to judge, if thou dost not wish to err." Thus too, in the things of nature, a reason can be assigned, since primary matter is altogether uniform, why one part of it was fashioned by God from the beginning under the form of fire, another under the form of earth, that there might be a diversity of species in things of nature. Yet why this particular part of matter is under this particular form, and that under another, depends upon the simple will of God; as from the simple will of the artificer it depends that this stone is in part of the wall, and that in another; although the plan requires that some stones should be in this place, and some in that place. Neither on this account can there be said to be injustice in God, if He prepares unequal lots for not unequal things. This would be altogether contrary to the notion of justice, if the effect of predestination were granted as a debt, and not gratuitously. In things which are given gratuitously, a person can give more or less, just as he pleases (provided he deprives nobody of his due), without any infringement of justice. This is what the master of the house said: "Take what is thine, and go thy way. Is it not lawful for me to do what I will?" (Matthew 20:14-15). (I:23:5)
Second, from the Summa Contra Gentiles (Book III, #160-164):
160
SINCE no one can be set on the way to his last end without the aid of divine grace, or without it have the necessary means of reaching that end, as are faith, hope, love and perseverance, some might think that man is not to blame for being destitute of these gifts, especially seeing that he cannot merit the assistance of divine grace, nor be converted to God unless God convert him: for none is responsible for that which depends on another. But allow this, and many absurdities follow. It follows that the man who has neither faith nor hope nor love of God, nor perseverance in good, still does not deserve punishment: whereas it is expressly said: "He that believeth not the Son shall not see life, but the wrath of God abideth on him" (John iii, 36). And since none reaches the end of happiness without the aforesaid endowments, it would follow further that there are some who neither attain to happiness nor yet suffer punishment of God: the contrary whereof is shown from what will be said to all present at the judgement of God: "Come . . . . possess ye the kingdom prepared for you, or, Depart . . . . into everlasting fire" (Matt. xxv, 34-41).
To solve this doubt, we must observe that though one can neither merit divine grace beforehand, nor acquire it by movement of his free will, still he can hinder himself from receiving it: for it is said of some: "They have said unto God, ‘Depart from us, we will not have the knowledge of thy ways’" (Job xxi, 14). And since it is in the power of free will to hinder the reception of divine grace or not to hinder it, not undeservedly may it be reckoned a man’s own fault, if he puts an obstacle in the way of the reception of grace. For God on His part is ready to give grace to all men: "He wills all men to be saved and to come to the knowledge of the truth" (1 Tim. ii, 4). But they alone are deprived of grace, who in themselves raise an obstacle to grace. So when the sun lights up the world, any evil that comes to a man who shuts his eyes is counted his own fault, although he could not see unless the sunlight first came in upon him.
161
WHEN it is said that it is in the power of free will to avoid putting obstacles to grace, that saying is to be understood of those in whom the natural faculty is unimpaired by sin. But if the will has fallen into evil courses by some previous inordinate act, it will not be altogether in its power to avoid putting obstacles in the way of grace. For though for some momentary occasion it may abstain from some particular act of sin by its own power, nevertheless, if left long to itself, it will fall into sin; and by sin an obstacle is put to grace. For when the mind of man turns aside from the state of righteousness, it clearly puts itself out of relation with its due end. Thus what ought to be the prime object of its affections, as being its last end, comes to be less loved than that other object to which it has inordinately turned, making of it another last end. Whatever in such a posture of the mind occurs to fit in with the inordinate end, however inconsistent with the due end, will be chosen, unless the will be brought back to due order, so as to prefer the due end to all others, and that is an effect of grace. But the choice of anything inconsistent with the last end puts an obstacle in the way of grace, as grace goes to turn one in the direction of the end. Hence after sin a man cannot abstain from all further sin before by grace he is brought back to due order.
Moreover, when the mind is inclined to a thing, it is no longer impartial between two alternatives. And that to which the mind is more inclined it chooses, unless by a rational discussion, not unattended with trouble, it is withdrawn from taking that side: hence sudden emergencies afford the best sign of the inward bent of the mind. But it is impossible for the mind of man to be so continually watchful as rationally to discuss whatever it ought to do or not to do. Consequently the mind will at times choose that to which it is inclined by the present inclination: so, if the inclination be to sin, it will not stand long clear of sin, thereby putting an obstacle in the way of grace, unless it be brought back to the state of righteousness.
Further we must consider the assaults of passion, the allurements of sense, the endless occasions of evil-doing, the ready incitements of sin, sure to prevail, unless the will be withheld from them by a firm adherence to the last end, which is the work of grace.
Hence appears the folly of the Pelagian view, that a man in sin can go on avoiding further sins without grace. On the contrary the Lord bids us pray: "Lead us not into temptation, but deliver us from evil."
But though persons in sin cannot of their own power help putting obstacles in the way of grace, unless they be forestalled by some aid of grace, still this lack of power is imputable to them for a fault, because it is left behind in them by a fault going before; as a drunken man is not excused from murder, committed in drunkenness, when he gets drunk by fault of his own. Besides, though this person in sin has it not in his unaided power altogether to avoid sin, still he has power here and now to avoid this or that sin: hence whatever he commits, he voluntarily commits, and the fault is imputed to him not undeservedly.
162
THOUGH the sinner raises an obstacle to grace, and by the exigence of the order of things ought not to receive grace, nevertheless, inasmuch as God can work setting aside the connatural order of things, as when He gives sight to the blind, or raises the dead, He sometimes out of the abundance of His goodness forestalls by the assistance of His grace even those who raise an obstacle to it, turning them away from evil and converting them to good. And as He does not give sight to all the blind, nor heal all the sick, that in those whom He heals the work of His power may appear, and in the others the order of nature may be observed, so He does not forestall by His aid all who hinder grace, to their turning away from evil and conversion to good, but some He so forestalls, wishing in them His mercy to appear, while in others He would have the order of justice made manifest. Hence the Apostle says: "God, though willing to show his wrath, and to make his power known, endured with much longsuffering vessels of wrath, fitted for destruction, that he might show forth the riches of his glory upon the vessels of mercy, which he hath prepared unto glory" (Rom. ix, 22, 23).
But when, of men who are enthralled in the same sins, God forestalls and converts some, and endures, or permits, others to go their way according to the order of things, we should not enquire the reason why He converts these and not those: for that depends on His sheer will, just as from His sheer will it proceeded that, when all things were made out of nothing, some things were made in a position of greater advantage than others (digniora). Hence again the apostle says: "Hath not the potter power over the clay, to make of the same lump one vessel unto honour and another unto dishonour?" (Rom. ix, 21.)
Hereby is refuted the error of Origen, who said that the reason why some were converted to God, and not others, was to be sought in divers works that their souls had done before they were united with their bodies, a theory already set aside (B. II, Chapp. XLIV, LXXXIII).
163
THOUGH there are some sinners whom God does not convert to Himself, but leaves them in their sins according to their deserts, still He does not induce them to sin.
1. Men sin by deviating from God their last end. But as every agent acts to its own proper and befitting end, it is impossible for God’s action to avert any from their ultimate end in God.
2. Good cannot be the cause of evil, nor God the cause of sin.
3. All the wisdom and goodness of man is derived from the wisdom and goodness of God, being a likeness thereof. But it is repugnant to the wisdom and goodness of man to make any one to sin: therefore much more to divine wisdom and goodness.
4. A fault always arises from some defect of the proximate agent, not from any defect of the prime agent. Thus the fault of limping comes from some defect of the shin-bone, not from the locomotor power, from which power however is whatever perfection of movement appears in the limping. But the proximate agent of human sin is the will. The sinful defect then is from the will of man, not from God, who is the prime agent, of whom however is whatever point of perfect action appears in the act of sin.
Hence it is said: "Say not, He himself hath led me astray: for he hath no use for sinful men: He hath commanded none to do impiously, and he hath not given to any man license to sin" (Ecclus xv, 12, 21): "Let none, when he is tempted, say that he is tempted by God: for God tempteth no man to evil" (James i, 13).
Still there are passages of Scripture, from which it might seem that God is to some men the cause of sin. Thus it is said: "I have hardened the heart of Pharaoh and his servants" (Exod. x, 1): "Blind the heart of this people, and make its ears dull, and close its eyes, lest perchance it see with its eyes, and be converted, and I heal it: Thou hast made us wander from thy ways: Thou hast hardened our heart, that we should not fear thee" (Isai. vi, 10: lxiii, 17): "God delivered them over to a reprobate sense, to do those things which are not seemly" (Rom. i, 28). All these passages are to be understood as meaning that God does not bestow on some the help for avoiding sin which He bestows on others. This help is not merely the infusion of grace, but also an exterior guardianship, whereby the occasions of sin are providentially removed from a man’s path. God also aids man against sin by the natural light of reason, and other natural goods that He bestows on man. When then He withdraws these aids from some, as their conduct deserves that he should, according to the exigency of His justice, He is said to harden them, or to blind them.
164
SINCE it has been shown that by the action of God some are guided to their last end with the aid of grace, while others, bereft of that same aid of grace, fall away from their last end; and at the same time all things that are done by God are from eternity foreseen and ordained by His wisdom, as has also been shown, it needs must be that the aforesaid distinction of men has been from eternity ordained of God. Inasmuch therefore as He has from eternity pre-ordained some to be guided to the last end, He is said to have ‘predestined’ them. Hence the Apostle says: "Who hath predestined us to the adoption of sons, according to the purpose of his will" (Eph. i, 5). But those to whom from eternity He has arranged not to give grace, He is said to have ‘reprobated,’ or ‘hated,’ according to the text: "I have loved Jacob, and hated Esau" (Malach. i, 2). In point of this distinction, inasmuch as some He has reprobated and some He has predestined, we speak of the divine ‘election,’ of which it is said: "He hath elected us in him before the constitution of the world" (Eph. i, 4). Thus it appears that predestination and election and reprobation is a part of divine providence, according as by the said providence men are guided to their last end. And it may be shown that predestination and election do not induce necessity, by the same arguments whereby it was shown that divine providence does not take away contingency from creation (Chap. LXXII).
But that predestination and election have no cause in any human merits may be shown, not only by the fact that the grace of God, an effect of predestination, is not preceded by any merits, but precedes all merit, but also by this further fact, that the divine will and providence is the first cause of all things that are made. Nothing can be cause of the will and providence of God; although of the effects of providence, and of the effects of predestination, one effect may be cause of another. "For who hath first given to him, and recompense shall be made him? For of him and by him and in him are all things: to him be glory forever, Amen" (Rom. xi, 35, 36).
For more, see here.
Published on the feast of St. Katherine Drexel.