This is the one Church of Christ which in the Creed is professed as one, holy, catholic and apostolic, which our Saviour, after His Resurrection, commissioned Peter to shepherd, and him and the other apostles to extend and direct with authority, which He erected for all ages as "the pillar and mainstay of the truth". This Church constituted and organized in the world as a society, subsists in the Catholic Church, which is governed by the successor of Peter and by the Bishops in communion with him, although many elements of sanctification and of truth are found outside of its visible structure. These elements, as gifts belonging to the Church of Christ, are forces impelling toward catholic unity.
Unitatis Redintegratio, another Vatican II document, goes into more detail (footnotes removed):
Even in the beginnings of this one and only Church of God there arose certain rifts, which the Apostle strongly condemned. But in subsequent centuries much more serious dissensions made their appearance and quite large communities came to be separated from full communion with the Catholic Church - for which, often enough, men of both sides were to blame. The children who are born into these Communities and who grow up believing in Christ cannot be accused of the sin involved in the separation, and the Catholic Church embraces upon them as brothers, with respect and affection. For men who believe in Christ and have been truly baptized are in communion with the Catholic Church even though this communion is imperfect. The differences that exist in varying degrees between them and the Catholic Church - whether in doctrine and sometimes in discipline, or concerning the structure of the Church - do indeed create many obstacles, sometimes serious ones, to full ecclesiastical communion. The ecumenical movement is striving to overcome these obstacles. But even in spite of them it remains true that all who have been justified by faith in Baptism are members of Christ's body, and have a right to be called Christian, and so are correctly accepted as brothers by the children of the Catholic Church.
Moreover, some and even very many of the significant elements and endowments which together go to build up and give life to the Church itself, can exist outside the visible boundaries of the Catholic Church: the written word of God; the life of grace; faith, hope and charity, with the other interior gifts of the Holy Spirit, and visible elements too. All of these, which come from Christ and lead back to Christ, belong by right to the one Church of Christ.
The brethren divided from us also use many liturgical actions of the Christian religion. These most certainly can truly engender a life of grace in ways that vary according to the condition of each Church or Community. These liturgical actions must be regarded as capable of giving access to the community of salvation.
It follows that the separated Churches and Communities as such, though we believe them to be deficient in some respects, have been by no means deprived of significance and importance in the mystery of salvation. For the Spirit of Christ has not refrained from using them as means of salvation which derive their efficacy from the very fullness of grace and truth entrusted to the Church.
Nevertheless, our separated brethren, whether considered as individuals or as Communities and Churches, are not blessed with that unity which Jesus Christ wished to bestow on all those who through Him were born again into one body, and with Him quickened to newness of life - that unity which the Holy Scriptures and the ancient Tradition of the Church proclaim. For it is only through Christ's Catholic Church, which is "the all-embracing means of salvation," that they can benefit fully from the means of salvation. We believe that Our Lord entrusted all the blessings of the New Covenant to the apostolic college alone, of which Peter is the head, in order to establish the one Body of Christ on earth to which all should be fully incorporated who belong in any way to the people of God. This people of God, though still in its members liable to sin, is ever growing in Christ during its pilgrimage on earth, and is guided by God's gentle wisdom, according to His hidden designs, until it shall happily arrive at the fullness of eternal glory in the heavenly Jerusalem.
Since God works outside the organizational structures of the Catholic Church, and since the Church and the Spirit of God are found in other Christian communities and traditions, the heritage of these other communities and traditions is important as constituting a part of the diversity of the Catholic Church. God gives diverse spiritual gifts to his Church, and the different members and parts of the Church are supposed to enrich the entire body by sharing their unique talents, gifts, experiences, skills, histories, cultures, etc. When any part of the experience of any part of the Body of Christ is lost to the larger body, the larger body is impoverished, missing out on something that God has given to it to enrich it.
Because of this, the Catholic Church talks about the "patrimony" possessed by other Christian communities and traditions--that is, the heritage that these groups have attained by means of their living of the life of the Church in the world through their unique histories. The Catholic Church wants to bring in all the separated brethren and unite them in full communion with the Catholic Church, but it doesn't want them to "convert" in such a way as to lose what they had gained in their previous traditions. It would rather that these traditions bring back into full communion with the Church the good parts of their heritage, thereby enriching the whole Catholic body.
Sometimes the Church takes particular steps to help churches and people to come into full communion while preserving their own patrimony. A good example of this is what the Church has done for Anglican individuals and churches who want to come into full communion. The Church has established what it calls "The Personal Ordinariate of the Chair of St. Peter." It is designed to help "those of the Anglican heritage entering full communion with the Catholic Church while maintaining distinctive elements of their theological, spiritual, and liturgical patrimony." (You can read more about it at the link provided above.)
This hasn't been nearly as widely discussed, but I think we need to work to preserve the Calvinist patrimony in the Catholic Church as well. The Reformed tradition has a rich history, forms of worship, particular forms of piety, theological emphases, etc., that are very much worth preserving. Here are some examples:
Reformed piety has produced many hymns, such as the hymns of John Newton ("Amazing Grace," etc.), or Isaac Watts, or Horatius Bonar. Many of these are already found in Catholic books of hymns, as I have observed from leafing through the one used in my parish and hearing, to my delight, some of them sung from time to time.
The Reformed tradition has emphasized the singing of psalms, while many other traditions have abandoned this practice to a great degree. Again, this is something that already resonates with Catholics, who also have a tradition that includes regular singing of psalms. The Reformed have developed a way of putting psalms into metrical form which is (at least in my experience) somewhat unique and can further enrich the Catholic tradition. Here's a classic example from the seventeenth century.
There is a strong emphasis on the reading and study of Scripture in the Reformed tradition, and being willing to follow it closely and carefully. I've not seen a study, but I'm sure that Reformed Christians tend to be more biblically literate on average than Christians in many other traditions.
The Reformed tradition has always had a strong emphasis on the sovereignty of God in general and the sovereign grace of God in salvation. They have always been strong promoters of understanding God's sovereign predestination over all of history and the efficaciousness of God's grace which saves us in spite of our own sinful hearts that would forever resist God and land us in hell unless he changed them from hearts of stone to hearts of flesh. There is a kind of piety that flows from this that focuses on trust in God's providential care over our lives; a recognition of our helplessness as mere creatures and of our sinful, fallen condition that would leave us hopeless apart from God's free, unmerited mercy; and praise and gratitude for God's infinite, amazing mercy to us in Christ. There is a recognition, as John Newton's "Amazing Grace" puts it, that "'tis grace has brought me safe thus far, and grace will lead me home." This piety shows up in some of the great hymns treasured by Reformed people, such as John Stocker's "Thy Mercy My God." Here are a couple of stanzas from the song as found on a Calvinist evangelical music website called, in quite typical Calvinist fashion, "Indelible Grace":
1. Thy mercy, my God, is the theme of my song,
The joy of my heart. and the boast of my tongue;
Thy free grace alone, from the first to the last,
Hath won my affections, and bound my soul fast.
3. Thy mercy is more than a match for my heart,
Which wonders to feel its own hardness depart;
Dissolved by Thy goodness, I fall to the ground,
And weep to the praise of the mercy Ive found.
Another good example is John Newton's hymn, "I asked the Lord that I Might Grow":
1 I asked the Lord that I might grow
In faith, and love, and every grace;
Might more of his salvation know,
And seek more earnestly his face.
2 'Twas he who taught me thus to pray;
And he, I trust, has answered prayer:
But it has been in such a way
As almost drove me to despair.
3 I hoped that in some favored hour,
At once he'd grant me my request;
And, by his love's constraining power,
Subdue my sins, and give me rest.
4 Instead of this, he made me feel
The hidden evils of my heart,
And let the angry powers of hell
Assault my soul in every part.
5 Yea more, with his own hand he seemed
Intent to aggravate my woe;
Crossed all the fair designs I schemed,
Blasted my gourds, and laid me low.
6 Lord, why is this? I trembling cried;
Wilt thou pursue thy worm to death?
'Tis in this way, the Lord replied,
I answer prayer for grace and faith.
7 These inward trials I employ
From self and pride to set thee free,
To break thy schemes of worldly joy,
That thou mayst seek thy all in me.
The Reformed tradition has a lot in common with the Catholic tradition, especially when compared with the broader evangelical world, such as a strong emphasis on the visibility of the church--that is, the church not just as a loose association of converted people but as a visible, formal institution with appointed shepherds, real authority, ecclesiastical discipline, sacraments, etc. There were many things along these lines that I did not understand in my pre-Reformed days that I learned during my sojourn with the Reformed tradition. There is also a deep concern, especially among the Presbyterians, for the unity of the visible church.
The Reformed tradition has a rich heritage of not only biblical exegesis and interpretation but also systematic theology. It's forms of scholastic, systematic theology as found in its greatest teachers, such as the great Francis Turretin of the seventeenth century, have an enormous amount in common with Catholic scholastic theology as that developed in the Middle Ages and is exemplified by great teachers like St. Thomas Aquinas. A Catholic well versed in this kind of theology would find himself surprisingly well at home reading much of Turretin.
There are great Reformed authors from whom a lot can be learned and who can function as very helpful theological and pastoral counselors. I mentioned Francis Turretin already for scholastic and philosophical theology. The great Puritan divines of the sixteenth and seventeenth centuries, such as William Perkins, William Ames, John Owen, and many others, are rich with biblical and pastoral insight. The commentaries of John Calvin himself are a great source of pastoral insight. One of my favorite authors of all time is Jonathan Edwards, from whom I have greatly profited.
Of course, the Reformed tradition, not being fully Catholicm has problems: It teaches Sola Scriptura, which wrenches the Bible out from its proper context in the Tradition of the Church and leads to an endless, growing cacophony of unchecked private interpretations that lead to endless disputes and church splits. (See here to get a sense of this--granted that not all of this confusion is due to Sola Scriptura, but a good bit of it no doubt is.) I mentioned above that the Reformed tradition has always emphasized the visible unity of the Church, but in practice this emphasis has been much watered down as Reformed people have realized that they just can't pull it off (because of Sola Scriptura, but they don't realize this). Now they tend to talk of worldwide catholic unity as something to be hoped for in the future Millennial kingdom, and most of them have given up much serious hope of seeing it in this world. The Reformed tradition, again because of Sola Scriptura, has severely impoverished the worship of the Church. Out of a sincere zeal to avoid man-made traditions, the most consistent of them have opposed all holidays outside of the Lord's Day, all rituals that can't be shown from Scripture, images, sacramentals, etc. The result is a worship that does not fully meet the needs of human nature (which is why the less strict Reformed churches have been slowly bringing these kinds of things back in over the past couple of centuries). The Catholic Church teaches that God has given his Spirit to his Church, and that the Spirit has guided the Church in the development of its piety over the centuries so that a rich heritage has developed. But the more strict Reformed, not trusting in this kind of guidance, and trying to stick only to what private individuals can get out of the Bible, have refused to allow most of these developments, fearing to add to the Word of God.
Other issues could be raised as well, and they should not be forgotten, but neither should we forget the great positives of the Reformed tradition. God has been at work among the Reformed churches, and there is much in that patrimony that can enrich the Catholic Church and which deserves to be preserved and promoted. I am very grateful to God for my sojourn in the Reformed faith. It caused me some difficulties, especially towards the end (as my theology came more and more to outgrow its limitations), but it also nourished me and helped me grow in Christ in many wonderful ways.
For more information on what the Reformed faith looks like, this is a nice, succinct pamphlet from the Orthodox Presbyterian Church. You might also check out some of the historic Reformed confessions, such as the Westminster Confession, the Belgic Confession, and the Heidelberg Catechism. Or check out some of the authors I mentioned above, most of whom you can find online.
Published on the feast of Sts. Marcellinus and Peter