While I am reluctant to prolong the debate about the terms 'is' and 'subsists in', I feel I ought to respond briefly to Peter Howard (December-January AD2000).
As I see it there are two views on what the Church believes about herself. The first is given by Pope Pius XII in his encyclical, Mystici Corporis Christi, which states (13) that 'this true Church of Jesus Christ is the One, Holy Catholic and Apostolic and Roman Church, the Mystical Body of Christ'.
The second view is supplied by the Dogmatic Constitution Lumen Gentium (no 8) which states that the Church of Christ 'subsists' in the Catholic Church. The Congregation for the Doctrine of the Faith recently commented on the term 'subsistence'. It is said to be an affirmation that the Church of Christ is concretely found in the Catholic Church as the latter embodies historical continuity and the permanence of all the elements instituted by Christ.
Thus we have two propositions. The first proposition, 'A (Church of Christ) is B (Catholic Church)', states that A and B are the same. The term 'is' states that the subject is, or is something.
From the second proposition 'A is concretely found in B' it may not be inferred that A is the same as B. 'To be found in' suggests a mode of being or existence in a certain manner or state, but not identity. Considering the two propositions, the conclusion is that the term 'subsists in' is not the same as the term 'is'.
In the words of St Joan of Arc: 'About Jesus Christ and the Church, I simply know that they are just one thing, and we shouldn't complicate the matter' (CCC par 795).