St. Augustine refers to John 19:34, where Christ’s side is pierced by the soldier’s sword and water and blood flowed out:
Here was opened wide the door of life, from which the sacraments of the Church have flowed out, without which there is no entering in unto life which is true life . . . Here the second Adam with bowed head slept upon the cross, that thence a wife might be formed of him, flowing from his side while he slept. O death, by which the dead come back to life! is there anything purer than this blood, any wound more healing!" (On the Gospel of John 120:2)
". . . The blood and water that flowed from the pierced side of the crucified Jesus are types of baptism and the Eucharist, the sacraments of new life. From then on, it is possible ‘to be born of water and the Spirit’ in order to enter the Kingdom of God" (CCC 1225).