His statement may seem unusual because, for us, what He says is the final word on any matter. He is the Faithful and True Witness (Revelation 3:14). For us, His testimony outweighs everything. But the ESV suggests a word that helps to make His meaning clear. It says, If I alone bear witness about [M]yself, [M]y testimony is not true. In other words, within the context, if His witness about Himself were the only witness, it would not have been accepted by those who were accusing Him. On a human level, Jesus acknowledges that something additional was required for even His own witness to be a valid testimony.
