Jamieson, Fausset and Brown on 1 John 5:8

8. agree in one--"tend unto one result"; their agreeing testimony to Jesus' Sonship and Messiahship they give by the sacramental grace in the water of baptism, received by the penitent believer, by the atoning efficacy of His blood, and by the internal witness of His Spirit (1Jo 5:10): answering to the testimony given to Jesus' Sonship and Messiahship by His baptism, His crucifixion, and the Spirit's manifestations in Him (see on 1Jo 5:6). It was by His coming by water (that is, His baptism in Jordan) that Jesus was solemnly inaugurated in office, and revealed Himself as Messiah; this must have been peculiarly important in John's estimation, who was first led to Christ by the testimony of the Baptist. By the baptism then received by Christ, and by His redeeming blood-shedding, and by that which the Spirit of God, whose witness is infallible, has effected, and still effects, by Him, the Spirit, the water, and the blood, unite, as the threefold witness, to verify His divine Messiahship [Neander].