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 is he that came by water and blood - Jesus was attested to be the Son of God and promised Messiah by water, i.e. his baptism, when the Spirit of God came down from heaven upon him, and the voice from heaven said, This is my beloved Son, in whom I am well pleased. Jesus Christ came also by blood. He shed his blood for the sins of the world; and this was in accordance with all that the Jewish prophets had written concerning him. Here the apostle says that the Spirit witnesses this; that he came not by water only - being baptized, and baptizing men in his own name that they might be his followers and disciples; but by blood also - by his sacrificial death, without which the world could not be saved, and he could have had no disciples. As, therefore, the Spirit of God witnessed his being the Son of God at his baptism, and as the same Spirit in the prophets had witnessed that he should die a cruel, yet a sacrificial, death; he is said here to bear witness, because he is the Spirit of truth.
Perhaps St. John makes here a mental comparison between Christ, and Moses and Aaron; to both of whom he opposed our Lord, and shows his superior excellence. Moses came by water - all the Israelites were baptized unto him in the cloud and in the sea, and thus became his flock and his disciples; 1 Corinthians 11:1, 1 Corinthians 11:2. Aaron came by blood - he entered into the holy of holies with the blood of the victim, to make atonement for sin. Moses initiated the people into the covenant of God by bringing them under the cloud and through the water. Aaron confirmed that covenant by shedding the blood, sprinkling part of it upon them, and the rest before the Lord in the holy of holies. Moses came only by water, Aaron only by blood; and both came as types. But Christ came both by water and blood, not typically, but really; not by the authority of another, but by his own. Jesus initiates his followers into the Christian covenant by the baptism of water, and confirms and seals to them the blessings of the covenant by an application of the blood of the atonement; thus purging their consciences, and purifying their souls.
Thus, his religion is of infinitely greater efficacy than that in which Moses and Aaron were ministers. See Schoettgen.
It may be said, also, that the Spirit bears witness of Jesus by his testimony in the souls of genuine Christians, and by the spiritual gifts and miraculous powers with which he endowed the apostles and primitive believers. This is agreeable to what St. John says in his gospel, John 15:26, John 15:27 : When the Comforter is come, the Spirit of truth, which proceedeth from the Father, he shall testify of me; and ye also shall bear witness, because ye have been with me from the beginning. This place the apostle seems to have in his eye; and this would naturally lead him to speak concerning the three witnesses, the Spirit, the Water, and the Blood, 1 John 5:8.