The Christian faith is grounded on the Word of God. This is what places it in the sovereign degree of certainty, as having the warrant of that eternal and infallible Truth. Faith which rests on anything else is not Christian. Therefore, the Word of God is the true rule of right-believing, as ground and rule are in this case one and the same thing.
Since this rule does not regulate our faith save when it is applied, proposed and declared, and since this may be done well or ill,--therefore it is not enough to know that the Word of God is the true and infallible rule of right-believing, unless I know what Word is God's, where it is, who has to propose, apply, and declare it. It is useless for me to know that the Word of God is infallible, and for all this knowledge I shall not believe that Jesus is the Christ, Son of the living God, unless I am certified that this Word is revealed by the heavenly Eather (sic): and even when I come to know this I shall not be out of doubt if I do not know how this is to be understood,--whether of an adoptive filiation in the Arian sense, or a natural filiation in the Catholic.
There is need, then, besides this first and fundamental rule the Word of God, of another, a second rule, by which the first may be rightly and duly proposed, applied, and declared. And in order that we may not be subject to hesitation and uncertainty, it is necessary not only that the first rule, namely, the Word of God, but also the second, which proposes and applies this rule, be absolutely infallible; otherwise we shall always remain in suspense and in doubt as to whether we are not being badly directed and supported in our faith and belief, not now by any defect in the first rule, but by error and defect in the proposition and application thereof. Certainly the danger is equal,--either of getting out of rule for want of a right rule, or getting out of rule for want of a regular and right application of the rule itself. But this infallibility which is required as well in the rule as in its application, can have its source only in God, the living and original fountain of all truth. Let us proceed.
Now as God revealed his Word, and spoke, or preached, by the mouth of the Fathers and Prophets, and at last by his own Son, then by the Apostles and Evangelists, whose tongues were but as the pens of scribes writing rapidly, God thus employing men to speak to men; so to propose, apply, and declare this his Word, he employs his visible Spouse as his mouthpiece and the interpreter of his intentions. It is God then who rules over Christian belief, but with two instruments, in a double way: (1) by his Word as by a formal rule; (2) by his Church as by the hand of the measurer and rule-user. Let us put it thus: God is the painter, our faith the picture, the colours are the Word of God, the brush is the Church. Here then are two ordinary and infallible rules of our belief: the Word of God, which is the fundamental and formal rule; the Church of God, which is the rule of application and explanation.
I consider in this second part both the one and the other, but to make my exposition of them more clear and more easy to handle, I have divided these two rules into several, as follows.
The Word of God, the formal rule of our faith, is either in Scripture or in Tradition. I treat first of Scripture, then of Tradition.
The Church, the rule of application, expresses herself either in her universal body by a general belief of all Christians, or in her principal and nobler parts by a consent of her pastors and doctors; and in this latter way it is either in her pastors assembled in one place and at one time, as in a general council, or in her pastors divided as to place and time, but assembled in union and correspondence of faith; or, in fine, this same Church expresses herself and speaks by her head-minister.* And these are four explaining and applying rules of our faith;--the Church as a whole, the General Council, the consent of the Fathers, the Pope.
Other rules than these we are not to seek; these are enough to steady the most inconstant. But God, who takes pleasure in the abundance of his favours, wishing to come to the help of the weakness of men, goes so far as to add sometimes to these ordinary rules (I refer to the establishment and founding of the Church) an extraordinary rule, most certain and of great importance,--namely, miracles--an extraordinary testimony of the true application of the Divine Word.
Lastly, natural reason may also be called a rule of right-believing, but negatively and not affirmatively. For if any one should speak thus: such a proposition is an article of faith, therefore it is according to natural reason:--this affirmative consequence would be badly drawn, since almost all our faith is outside of and above our reason. But if he were to say: this is an article of faith, therefore it cannot be against natural reason:--the consequence is good. For natural reason and faith, being supported on the same principles, and starting from one same author, cannot be contrary to each other.
Here then are eight rules of faith: Scripture, Tradition, the Church, Councils, the Fathers, the Pope, miracles, natural reason. The two first are only a formal rule, the four following are only a rule of application, the seventh is extraordinary, and the eighth negative. Or, he who would reduce all these rules to a single one, would say that the sole and true rule of right-believing is the Word of God preached by the Church of God.