1 John 5:6

This is he that came by water and blood, even Jesus Christ; not with the water only, but with the water and with the blood.

From the times of Tertullian, the more discerning scholars have referred these words to the baptism of Christ in water (as the Greek reads here), and to his crucifixion (aptly described as his coming "in the blood"). Some have seen a reference here to John 19:34; and, as Bruce stated it, "I should not care to deny this."[11] It undoubtedly refers to all of these events; and, even beyond this, it undoubtedly suggested to the apostle the two grand ordinances of the Christian religion: baptism and the Lord's Supper, as indicated by his specific reference to them two verses later. However, the matter at hand in this verse related to the heresy of the Cerinthians who admitted that Jesus was the Christ after his baptism, but denied that he was the Christ in his crucifixion. Again from Bruce:

From their point of view, Christ came by water, but not by blood. Therefore, John emphasized that he came "not with water only, but with the water and with the blood"; with the clear meaning that Jesus was proclaimed as the Son of God as truly in his death as he was in his baptism.[12]

John's refutation of that heresy was as precise and devastating as any that could have been given.

[11] F. F. Bruce, Answers to Questions (Grand Rapids, Michigan: Zondervan Publishing House, 1972), p. 133.

[12] Ibid., p. 134.