John 19:34 - Matthew Poole's English Annotations on the Holy Bible

But one of the soldiers, to make sure of him, pierced his side, out of which it is said that there presently came forth blood and water. That there should come out blood is no wonder, nor yet that there should come forth water. Blood being congealed, it is ordinary to see water on the top of the vessel where it is. And besides, anatomists tell us, that in the hollow part of the breast there are watery as well as bloody humours in the membrane that encompasses the heart, which being pierced, and the water let out, the living creature dieth necessarily. But yet in regard of the next words, He that saw it bare record, and he knoweth that he saith true, &c., most divines think, that there was some mystery in this water and blood which came out of Christ's side pierced. Some would have the two sacraments of the gospel signifies by this water and blood. Christ is said to have come by water and blood, 1 John 5:6; that is, say interpreters, he brought in a true expiation of sins by his blood, and the laver of regeneration, washing the soul from its filthiness: and thus be proved the true Antitype, answering the Jewish types in sacrifices and divers washings.