67 Here, Venerable Brethren, We wish to speak in a very special way of our union with Christ in the Body of the Church, a thing which is, as Augustine justly remarks, sublime, mysterious and divine; ut for that very reason it often happens that many misunderstand it and explain it incorrectly. It is at once evident that this union is very close. In the Sacred Scriptures it is compared to the chaste union of man and wife, to the vital union of branch and vine, and to the cohesion found in our body. Even more, it is represented as being so close that the Apostle says: "He (Christ) is the Head of the Body of the Church," and the unbroken tradition of the Fathers from the earliest times teaches that the Divine Redeemer and the Society which is His Body form but one mystical person, that is to say to quote Augustine, the whole Christ. Our Savior Himself in His sacerdotal prayer did not hesitate to liken this union to that wonderful unity by which the Son is in the Father, and the Father in the Son.
Source: Mystici Corporis Christi (Vatican.va)