28 That He completed His work on the gibbet of the Cross is the unanimous teaching of the holy Fathers who assert that the Church was born from the side of our Savior on the Cross like a new Eve, mother of all the living. "And it is now," says the great St. Ambrose, speaking of the pierced side of Christ, "that it is built, it is now that it is formed, it is now that it is...molded, it is now that it is created... Now it is that arises a spiritual house, a holy priesthood." One who reverently examines this venerable teaching will easily discover the reasons on which it is based.
Source: Mystici Corporis Christi (Vatican.va)