Through The Coming of Jesus God's Son, Which Was Well Testified To, We Can Know That We Have Eternal Life (1 John 5:6-13).
‘This is he who came by water and blood, even Jesus Christ, not with the water only, but with the water and with the blood. And it is the Spirit who bears witness, because the Spirit is the truth. For there are three who bear witness, the Spirit, and the water, and the blood, and the three agree in one. If we receive the witness of men, the witness of God is greater, for the witness of God is this, that he has borne witness concerning his Son.'
Here John now establishes what he has been saying about Jesus the Son of God. Jesus Christ did not just come by water (either through His natural birth or more probably through His baptism), but through water and blood (through the water and blood that flowed from Him at His death - John 19:34). His natural birth/baptism was one way in which He presented Himself, but equally He presented Himself through His physical death. It was a tenet of various false teachers that ‘the Christ spirit' had come on the body of Jesus at His baptism and had left Him before His death. No, says John, He was the Christ in His death as well as in His life. It was the Christ Who died on the cross.
Indeed this is borne witness to by the Spirit, for He is the Spirit of truth. He came on Jesus with power at Jesus' baptism, where Jesus was testified to as the only Son and the Servant Who was pleasing to God, and He came to Him powerfully in His death when He raised Him from the dead (Romans 1:4). So all three agree in their witness to Jesus as the Christ, the Spirit, the water and the blood, and all are agreed together and are one in revealing Him as the Christ. In both His life and His death he was the Christ.
The Spirit further bears witness to Christ through God's witnesses, first the Apostles, then those whom the Apostles appointed, and then through the leaders of the true churches.
But God Himself is also the witness to His Son. He Himself bore witness, for it was he Who sent the Holy Spirit on Him at His baptism, and made His declaration of Who He was as His Son, and how pleasing He was as His Servant, and it was He Who powerfully raised Him from the dead through His Holy Spirit at His resurrection. And His witness is greater than any witness of man.
So if we accept the witness of men, the witness of those who knew Jesus and knew Him in His life and who saw these wonderful events, we must even more accept the witness of God Who not only gave Him His Holy Spirit, Who was both with Him in His baptism and in His death and resurrection, but has also from that time given Him the power to give life to whom He will. For God's witness is that by this He has borne witness to His Son.