MYSTICI CORPORIS CHRISTI
Encyclical of Pope Pius XII, published in 1943, on the Church as the Mystical Body of Christ. The Church is a body because she is a visible, living, and growing organism, animated by the Spirit of God. She is a mystical body because her essential nature is a mystery, and all her teachings, laws, and rites are sacramental sources of grace. And she is the mystical body of Christ because he founded the Church. He remains her invisible Head and through him all blessings are communicated to her members, and through them to the rest of humankind.