35 The first benefit which We trust the Church will reap from a deepened self-awareness, is a renewed discovery of its vital bond of union with Christ. This is something which is perfectly well known, but it is supremely important and absolutely essential. It can never be sufficiently understood, meditated upon and preached. What shall We not say about this truth, which is the principal item, surely, of the whole of our religious heritage? Fortunately, you already have an excellent grasp of this doctrine, and here We would add nothing further except to make a strenuous recommendation that you always attribute maximum importance to it and look upon it as a guiding principle both in your spiritual life and in your preaching of the word of God. Consider the words of Our Predecessor, Pius XII, rather than Our own. In his memorable encyclical Mystici Corporis he wrote: "We must accustom ourselves to see Christ Himself in the Church. For it is indeed Christ who lives in the Church, and through her teaches, governs, and sanctifies; and it is also Christ who manifests Himself in manifold guise in the various members of His society." (18) How gratifying and pleasant it is to dwell on the words of Sacred Scripture, the Fathers, Doctors and Saints, which come to our minds when we contemplate this wonderful article of faith. Was it not Jesus Himself who told us that He was the vine and we the branches? (l9) Do we not have before us all the riches of St. Paul's teaching, who never ceases to remind us that we "are all one person in Jesus Christ"? (20) He is always exhorting us to "grow up in him who is the head, even Christ, from whom the whole body...," (21) and admonishes us that "Christ is all in all." (22) As for the Doctors of the Church, We need only recall this passage from St. Augustine: ". . . Let us rejoice and give thanks that we have become not only Christians, but Christ. Do you understand, brothers, the grace of Christ our Head? Wonder at it, rejoice: we have become Christ. For if He is the Head, we are the members; He and we form the whole man . . . the fullness of Christ, therefore; the head and the members. What is the head and the members? Christ and the Church." (73)
Source: Ecclesiam Suam (Vatican.va)