I am one that bear witness of myself. — The Greek has no word to express the English “one.” It is more exactly, I am He who beareth witness. The twofold witness is (1) in His own individuality — I, who know whence I came and whither I go, testify of Myself to you who know neither; and (2) in the fact that the Father sent Him. He is the Ambassador from the Father, accredited by the Father’s works and the voice from heaven, and His word is official as well as personal.
The thought is closely connected with that of John 8:16. His judgment is not individual judgment, because of the union with the Father. His witness is not only individual witness, but that of the Father also. The whole passage should be carefully compared with the close of John 5.