Vatican II's Lumen Gentium 8 says:
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.(I changed the capitalization to match the Latin.) Much discussion has been expended on the claim that the Church in the world "subsists in" the Catholic Church. Some interpreters took this as a weakening of the traditional teaching that the Church of Christ is the Catholic Church. On this reading, the Church of Christ is a larger entity, and the Catholic Church is a part of it. A fairly recent Congregation for the Doctrine of the Faith (CDF) statement states that "subsists in" indicates "full identity".
I want to argue that the CDF is right on textual grounds (though the CDF is presumably also drawing on non-textual information about the intentions of the Council Fathers).
What does the mysterious phrase "subsists in" mean? Here we have to remember that at the time of Vatican II, the lingua franca of Catholic thought was Thomism. Even those who did not philosophically or theologically agree with St. Thomas would use thomistic vocabulary to express their views. The phrase "subsists in" is a scholastic phrase that Aquinas uses a number of times. Here are the claims I've found in my search of a lot of Aquinas' works (admittedly in English, but I am trusting the translators):
- God subsists in his essence
- The divine understanding subsists in itself
- The essence of the Father subsists in the Son
- Human beings subsist in their essences
- Christ subsists both in a human and a divine nature
- Things subsist in their being
- A substance subsists in its species
- The form of the angel or the separated soul subsists in the being
The phrase, thus, is compatible with identity as well with a very intimate relationship that isn't quite identity. We need to turn to context now. First, take the paragraph as a whole. The first sentence says that the "one Church of Christ" is "one, holy, catholic and apostolic". To read the text as saying that the Church of Christ is not identical with the Catholic Church is to attribute to the text the absurdity of saying that the "one, holy, catholic and apostolic" Church is not the catholic Church. One might try to distinguish between "catholic" in the sense of "universal" and "catholic" in a sense that indicates the Roman Catholic Church, but the text makes no such distinction, and it is fair to assume that the same word is used in the same sense in the same paragraph.
Next, take the phrase "although many elements of sanctification and of truth are found outside of its visible structure", which elements are described as "gifts belonging to the Church of Christ". This text, I think, does not fit well with the idea that the Church of Christ is the larger entity that includes the Catholic Church as a part of it. For if the Church of Christ were the larger entity, there would be no need to emphasize that the elements of sanctification and truth (I assume these are the many true and good things found in non-Catholic congregations) do in fact belong to the Church of Christ.
Moreover, the preceding paragraph introduced a distinction between the Church as a "society structured with hierarchical organs" and the "mystical Body of Christ", which two natures it says are not separate but form a complex entity, and are like the two natures of the incarnate Christ. The "society structured with hierarchical organs" surely is the Catholic Church. It cannot refer to some alleged larger Church of Christ that includes Protestants, since Protestants do not have hierarchical organs as Lumen Gentium understands them (Lumen Gentium understands the hierarchy as constituted primarily by the Pope and Bishops). But at the same time, this discussion of the Church as a structured society surely is the same entity as the Church of Christ "constituted and organized in the world as a society". Lumen Gentium's overall understanding of organic structure is hierarchical: the Pope is the principle of organic unity, and from him proceeds the unity of the Bishops.
So, the context leads us to accept that in the thought of Lumen Gentium the Church of Christ is the Catholic Church. But why, then, the use of "subsists in"? Here is a suggestion that may be completely wrong: Some of the operations of the Catholic Church (e.g., the sacrament of baptism) extend beyond the Catholic Church. Yet they always are the operations of the Church of Christ, and derive their efficacy from Christ's promises to the Catholic Church. It is not that the Church of Christ extends beyond the Catholic Church, but the operations of the Church of Christ do. And the use of "subsists in" makes possible such a distinction. This fits with the CDF's explanation, I think.