1 John 5:6

The faith of the Christian believer (or the believer in Christ) being thus mighty and victorious, it had need to be well founded, to be furnished with unquestionable celestial evidence concerning the divine mission, authority, and office of the Lord Jesus; and it is so; he brings his credentials along with him, and he brings them in a way by which he came and in the witness that attends him. I. In the way and manner by which he came; not barely by which he came into the world, but by and with which he came, and appeared, and acted, as a Saviour in the world: This is he that came by water and blood. He came to save us from our sins, to give us eternal life, and bring us to God; and, that he might the more assuredly do this, he came by, or with, water and blood. Even Jesus Christ; Jesus Christ, I say, did so; and none but he. And I say it again, not by or with water only, but by and with water and blood, Jo1 5:6. Jesus Christ came with water and blood, as the notes and signatures of the true effectual Saviour of the world; and he came by water and blood as the means by which he would heal and save us. That he must and did thus come in his saving office may appear by our remembering these things: - 1. We are inwardly and outwardly defiled. (1.

— Matthew Henry's Concise Commentary (public domain)

This--the Person mentioned in Jo1 5:5. This Jesus. he that came by water and blood--"by water," when His ministry was inaugurated by baptism in the Jordan, and He received the Father's testimony to His Messiahship and divine Sonship. Compare Jo1 5:5, "believeth that Jesus is the Son of God," with Joh 1:33-34, "The Spirit . . . remaining on Him . . . I saw and bare record that this is the Son of God"; and Jo1 5:8, below, "there are three that bear witness in earth, the Spirit, and the water, and the blood." Corresponding to this is the baptism of water and the Spirit which He has instituted as a standing seal and mean of initiatory incorporation with Him. and blood--He came by "the blood of His cross" (so "by" is used, Heb 9:12 : "by," that is, with, "His own blood He entered in once into the holy place"): a fact seen and so solemnly witnessed to by John. "These two past facts in the Lord's life are this abiding testimony to us, by virtue of the permanent application to us of their cleansing and atoning power." Jesus Christ--not a mere appellation, but a solemn assertion of the Lord's Person and Messiahship. not by, &c.--Greek, "not IN the water only, but IN the water and IN (so oldest manuscripts add) the blood." As "by" implies the mean through, or with, which He came: so "in," the element in which He came.

— Jamieson-Fausset-Brown Commentary (public domain)