John Calvin on John 5:31-32

31. If I testify concerning myself. He does not here take any thing away from the credit due to his testimony, which he elsewhere asserts in strong terms, but he speaks by way of concession; for Christ, having been in other respects most abundantly supported, consents that they should not believe his word. “If my testimony concerning myself,” says he, “is suspected by you according to the ordinary custom of men, let it go for nothing.” Now we know that what any man asserts about himself is not reckoned to be true and authentic, although in other respects he speak truth, because no man is a competent witness in his own cause. Though it would be unjust to reduce the Son of God to this rank, yet he prefers to surrender his right, that he may convince his enemies by the authority of God.

John 5:33-36

33. You sent to John, and he gave testimony to the truth. 34. But I receive not testimony from man; but these things I say that you may be saved. 35. He was a burning and shining lamp, and for a time you were willing to amuse yourselves in his light. 36. But I have greater testimony than that of John; for the works which my Father gave me to perform, the very works which I do, testify concerning me that the Father hath sent me.