To strengthen our overcoming faith John draws our attention to the mode of Christ’s coming. ‘He that came’ is shorthand for the incarnation referring to Christ’s pre-existence, and stressing the divine purpose that brought him into the world.
The mystery of the incarnation is so great that our hearts would stagger in unbelief unless God confirmed it by powerful witnesses. It is an essential element of the Christian Gospel that in Jesus Christ, God and man have been united in one person, but how could the infinite, eternal God humble himself to the extent that he took the nature of his own creature and joined himself to what is finite, mortal, and changeable? Although this is beyond our comprehension, without it there can be no communication between God and man, no atonement, and no hope for lost men and women.
Some see a reference to the water and blood that came from the spear wound in Jesus’ side, but the words ‘not by water only’ rule this out, for water and blood came out of his side together, and there could be no possibility of one without the other. Neither are the words an allegorical reference to baptism and the Lord’s Supper, for they speak of the nature of his coming not to the consequence of it. It may be that the word ‘water’ goes back further than Christ’s baptism and is used to refer to his physical birth, in agreement with Christ’s words in John 3:5-6, where the close parallel in those verses between being born of water and being born of the flesh will not allow the idea of baptism to be read into the passage. Here John may be using the same image, and arguing that Christ’s physical birth, foretold in the Old Testament, testifies to his true humanity, for no angel or spirit ever went through the process of physical birth. He experienced true birth and true death, both according to Scripture.