This is he that came by water and blood, even Jesus Christ; not by water only, but by water and blood. And it is the Spirit that beareth witness, because the Spirit is truth.
This - the Person mentioned, 1 John 5:5: Jesus.
He that came by water and blood - "by water," when His ministry was inaugurated by baptism in Jordan, and He received the Father's testimony to His Messiahship and divine Sonship. Compare 1 John 5:5, with John 1:33-34, and 1 John 5:8, below. Corresponding is the baptism of water and the Spirit, which He instituted as a standing seal of initiatory incorporation with Him (John 3:5). "And blood" He came by "the blood of His cross" (so "by," Hebrews 9:12; Hebrews 9:1:e., with) - a fact seen and solemnly witnessed to by John. 'These two past facts in the Lord's life are this abiding testimony to us, by the permanent application to us of their cleansing and atoning power' (Alford).
Jesus Christ - not a mere appellation, but a solemn assertion of the Lord's Person and Messiahship.
Not by. [Greek, 'not IN the water only, but IN the water and IN (B; but 'Aleph (') omits) the blood.'] As "by" х dia (G1223)] implies the mean through, or with, which He came, so 'in,' the element in which He came. 'The' implies their being the well-known symbols. John Baptist came only baptizing with water; therefore was not the Messiah. Jesus came first to undergo Himself the double baptism of water and blood, then to baptize us with the Spirit-cleansing, of which water is the sacramental seal, and with His atoning blood, the efficacy of which, once-for-all shed, is perpetual; and therefore is the Messiah. It was His shed blood which gave water-baptism its spiritual significancy. We are baptized into His death: the grand point of union between us and Him, and, through Him, between us and God.
It is the Spirit ... The Holy Spirit is an additional witness (1 John 5:7), besides the water and the blood, to Jesus' Sonship and Messiahship. The Spirit attested these at Jesus' baptism, by descending on Him, and throughout His ministry, by enabling Him to speak and do what man never could have spoken or done. It is the abiding Spirit that beareth witness of Christ-in the inspired New Testament, and in the hearts of believers, and in the spiritual reception of baptism and the Lord's Supper.
Because the Spirit is truth. It is His essential truth which gives His witness such infallible authority.