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The previous section has emphasized the importance of faith in the Son and, as in his gospel, John holds that faith depends on testimony, thus “the reasonableness of believing in Jesus is grounded upon the validity of the testimony which is borne to Him” (Stott, 176).
Water probably refers to Jesus' baptism and blood to his death. It emphasizes the unity of his life, in contrast to the false teaching that joined the eternal Christ with the human Jesus at his baptism but separated them before his death. This emphasis is at one with the early preachers in Acts who insisted that the glorified Christ was this same Jesus whom their audience had put to death (Acts 2:32). He was fully human and fully divine throughout his whole career; thus there is life in the Son.
This refers to the Spirit speaking through the Word, convincing the hearer of the truth of Jesus to which the Word bears witness (see note on 2:20-23).
(Persons familiar with the KJV will recognize that the witness of the heavenly Trinity is omitted. See any contemporary commentary for a full explanation of this exclusion. It has no valid manuscript support.) Here John collates the earthly witnesses (water and blood) with the witness of God (cf. Jn 5:36-37). The Spirit bore witness to the historical Jesus and continues to bear witness to his authenticity. No doubt the phrase here refers to the internal witness of God to the truth of the objective witness of the water and the blood. The result of accepting God’s testimony is life in the Son.