The Father by a voice from heaven testified of Christ, that he was his well beloved Son, in whom he was well pleased, Matthew 3:17. Some understand it of John the Baptist, of whom he speaketh, 1 Thessalonians 5:33. But he naming John in the next verse, it seems most proper to understand this of the Father testifying of Christ, both at his baptism, and also at his transfiguration; and to interpret the next verse, as speaking of another testimony distinct from that of John. And (saith our Saviour) I know, that is, I am fully assured, that his testimony of me is true; for God is that God who cannot lie, but is truth itself. So that I do not barely testify of myself; for my Father, whom you all own to be a God of truth, and who cannot lie, and whom know to be such, he testifieth of me, and none can contradict his testimony.
Related Commentaries of John 5:32
John 5:32
32 There is another that beareth witness of me; and I know that the witness which he witnesseth of me is true.