The Spirit, the Water, and the Blood.
I. Consider the testimony of the water. I believe that the reference here is exclusively to baptism the baptism of Jesus Himself, and probably also the baptism which He instituted, and which remains as a permanent ordinance in connection with His name. This is the testimony of the water. Jesus, the Christ, came not by water only; but He did come by water. He was baptised by John in the Jordan. The importance attached by the Evangelists to the baptism of Jesus is surely not without significance. It stands on the very threshold of Christ's public ministry. It was His initiation into that ministry. It was His own open consecration of Himself to His own great work in relation to the new era; and the signs which accompanied His baptism were, so to speak, the manifest anointing by the Father of His Son. Thus Jesus, the Christ, "came by water." His public ministry was inaugurated by a baptism, which brought with it a Divine testimony to His being the Anointed.
II. Consider the testimony of the blood. His was a baptism, not only of consecration, but of suffering. The blood-shedding of Jesus was really a testimony to His Divine Sonship; it was the price He was willing to pay for the world's redemption; it was the completion of His revelation of the Father. Not until He hung upon the cross could He say, "It is finished."
III. Consider the testimony of the Spirit. Even during His life on earth, the Spirit which manifestly shone through the character, and conduct, and works of Jesus Christ, bore witness to Him as the Anointed of the Father. But, again, this Spirit with which Jesus was anointed was a Spirit which He was also to impart. "The Spirit beareth witness" in the Church "because the Spirit is truth."
T. C. Finlayson, Christian World Pulpit,vol. xviii., p. 195.