26 As We set out briefly to expound in what sense Christ founded His social Body, the following thought of Our predecessor of happy memory, Leo XIII, occurs to Us at once: "The Church which, already conceived, came forth from the side of the second Adam in His sleep on the Cross, first showed Herself before the eyes of men on the great day of Pentecost." For the Divine Redeemer began the building of the mystical temple of the Church when by His preaching He made known His Precepts; He completed it when he hung glorified on the Cross; and He manifested and proclaimed it when He sent the Holy Ghost as Paraclete in visible form on His disciples.
Source: Mystici Corporis Christi (Vatican.va)