2 “Then came the soldiers, and broke the legs of the first, and of the other who was crucified with Him. But when they came to Jesus, and saw that He was dead already, they broke not His legs: but one of the soldiers with a spear laid open His side, and immediately came there out blood and water.” A suggestive word was made use of by the evangelist, in not saying pierced, or wounded His side, or anything else, but “opened;” that thereby, in a sense, the gate of life might be thrown open, from whence have flowed forth the sacraments of the Church, without which there is no entrance to the life which is the true life.
That blood was shed for the remission of sins; that water it is that makes up the health-giving cup, and supplies at once the laver of baptism and water for drinking. This was announced beforehand, when Noah was commanded to make a door in the side of the ark, whereby the animals might enter which were not destined to perish in the flood, and by which the Church was prefigured. Because of this, the first woman was formed from the side of the man when asleep, and was called Life, and the mother of all living. Truly it pointed to a great good, prior to the great evil of the transgression (in the guise of one thus lying asleep). This second Adam bowed His head and fell asleep on the cross, that a spouse might be formed for Him from that which flowed from the sleeper's side.
O death, whereby the dead are raised anew to life! What can be purer than such blood? What more health-giving than such a wound?
Source: Tractates on the Gospel of John (New Advent)