On our way homeward to God, Mother Church nourishes us by her sacraments and guides us by her teachings which provide doctrinal content for Christian living. There are doctrines that are timeless, but some teachings' relevance seems to vanish in course of time.
As an example, Pius XII identified the Church of Christ with the Catholic Church in his encyclicals Mystici Corporis Christi (1943) and Humani Generis (1950) reflecting the traditional teaching of the Church:
* 'If we would define and describe this true Church of Christ - which is the One, Holy, Catholic, Apostolic and Roman Church - we shall find nothing more noble, more sublime or more divine than the expression 'The Mystical Body of Christ'' (Mystici Corporis Christi, 13).
* 'Some say they are not bound by the doctrine explained in our encyclical letter a few years ago, and based on the Sources of Revelation, which teaches that the Mystical Body of Christ and the Roman Catholic Church are one and the same thing' (Humani Generis, 27).
Now time, a little over 10 years, brought a change. Pius XII stated that the Church of Christ is the Roman Catholic Church. This ontological definition meant that both have a common boundary.
Lumen Gentium introduced the subsistence formula, an essential element for ecumenical dialogue. Accordingly the Church of Christ now 'subsists' in the Catholic Church, i.e., has her concrete form of existing in the Catholic Church.
Identity of the latter has now shifted from its ontological basis and is now dependent on the affirmation that Christ's Church is 'concretely present' in the Catholic Church, as the latter embodies historical continuity and the permanence of all the elements instituted by Christ.
Although fully present only in the Catholic Church, nevertheless at the same time, beyond the boundary of the Catholic Church, Christ's Church is also present and operative in churches and ecclesial communities lacking full communion with the Catholic Church. Thus, as Fr Aidan Nichols observes, Christ's Church is not exhaustively coterminous with the Catholic body.
The above leads to the question of what truth-value an encyclical has which claims to be based on sources of Revelation? Is its teaching timeless or is it subject to an ever-changing time in which we all change, and with us our ideas and 'truths'?