A traditional descriptor of the Church is that She is the Mystical Body of Christ, in which are united Our Lord as Head and the faithful as members. This unity is what we make reference to when we profess in the Creed that the Church is One and Catholic. It is also how we may distinguish true ecumenism (which seeks to resolve the causes of doctrinal division, most particularly between Catholicism and Orthodoxy) from false ecumenism (which seeks to minimize or dismiss the substantive doctrinal differences between Catholicism and non-Catholic creeds).
As the specters of false ecumenism and self-destruction loomed over Christendom and the world in the middle of the twentieth century, Pope Pius XII issued the encyclical Mystici Corporis Christi in 1943 to address the doctrine of the Mystical Body and its implications. We reproduce excerpts from that encyclical beginning today, and ask the intercession of Our Lady, Help of Christians, that the division and confusion now rampant in Christ’s Body and in the world may be healed.
The doctrine of the Mystical Body of Christ, which is the Church,1 was first taught us by the Redeemer Himself. Illustrating as it does the great and inestimable privilege of our intimate union with so exalted a Head, this doctrine by its sublime dignity invites all those who are drawn by the Holy Spirit to study it, and gives them, in the truths of which it proposes to the mind, a strong incentive to the performance of such good works as are conformable to its teaching. For this reason, We* deem it fitting to speak to you on this subject through this Encyclical Letter, developing and explaining above all, those points which concern the Church Militant. To this We are urged not only by the surpassing grandeur of the subject but also by the circumstances of the present time.
For We intend to speak of the riches stored up in this Church which Christ purchased with His own Blood,2 and whose members glory in a thorn-crowned Head. The fact that they thus glory is a striking proof that the greatest joy and exaltation are born only of suffering, and hence that we should rejoice if we partake of the sufferings of Christ, that when His glory shall be revealed we may also be glad with exceeding joy.3
1 Cf. Col. I, 24.
2 Acts, XX, 28.
3 Cf. I Peter, IV, 13.
— Mystici Corporis Christi 1-2
* It is customary for a Pope to refer to himself in the first person plural, out of respect for the continuity and grave dignity of the Petrine office.