1 John 5:6

This is he that came by water and blood, [even] Jesus Christ,
&c.] By "water" is not meant the ablutions or washings of the ceremonial law; Christ came not by these, but to make an end of them; his blood, which cleanseth from all sin, being the antitype, and so the fulfilling end of them: nor the purity of his nature, life, and conversation; though he came into the world that holy thing which is called the Son of God; and was holy in his nature, and harmless in his life, and did no sin, and so was fit to be a sacrifice for the sins of others: nor does it intend the washing and cleansing of his people from their sins; this is what he came to do, and has done, and not what he came by: but the ordinance of water baptism is designed; and though Christ did not come baptizing with water, he having a greater baptism to administer, yet that he might be made manifest, John came baptizing in that way; and Christ, as the Son of God, came, or was made manifest by John as such, at the waters of Jordan, and at his baptism; there he was declared to be the Son of God by his Father's voice from heaven:

not by water only;
he did not come by water only, as Moses did, who was drawn out of it, and therefore so called; or as John, who came administering water baptism externally only:

but by water and blood;
by "blood" as well as water; by which is meant, not the blood of bulls and goats; Christ came to put an end unto, and lay aside the shedding of that blood; but his own blood is intended, and not reconciliation and atonement for the sins of his people, which was what he came to do, and has done, and not what he came by: but the sense is, that as at baptism, so at his sufferings and death, he was made manifest to be the Son of God; as he was to the centurion and others, that were with him, when they observed the earthquake, and the things that were done; and at his from the dead he was declared to be the Son of God with power: and this might be seen in the cleansing and atoning virtue of his blood, which is owing to his being the Son of God. There may be here an allusion to the water and blood which came out of his side, when pierced on the cross, which this Apostle John was an eyewitness of. Some copies add here, and in the former clause, "and by the Spirit"; as the Alexandrian copy, three of Beza's copies, and the Ethiopic version: but it seems unnecessary, since it follows,

and it is the Spirit that beareth witness;
by which may be meant, either the Gospel, which is the Spirit that gives life, and is so called, because by it the Spirit of God, in his gifts and graces, is received, and which is a testimony of the person, as well as of the offices, and grace of Christ; or rather those miraculous works which Christ did by the Spirit, to which he often appeals, as witnesses of his divine sonship, and equality with the Father, as well as of his being the true Messiah; or else the Holy Spirit, who bore testimony to Christ, by his descent on him at his baptism, and upon his apostles at the day of Pentecost, and by attending, succeeding, and confirming the Gospel, which is the testimony of him; and he is elsewhere, as well as here, and in the context, spoken of as a witness of Christ, ( Acts 5:32 ) ;

because the Spirit is truth;
he is the Spirit of truth, and truth itself; he is essentially truth; his testimony is most true, and firmly to be believed. The Vulgate Latin version reads, "because Christ is the truth".