“I am telling you now before it happens, so that when it does happen you will believe that I am He.
Even though He knows exactly who the traitor is, what he is going to do, and what the resulting violent action will be, there is not a trace of anger in the voice of Jesus, only sorrow. He is making one last appeal to the heart of Judas to repent.
The ability of one of their own to betray Jesus and to have His enemies gain the advantage over Him, might have destroyed the faith of the disciples had He not told them beforehand exactly what was going to happen. On the other hand, this expression of concern on the part of Jesus for His followers will not only tend to confirm their faith in Him, but also show them the deeper meaning in the reason Judas was even among them.
“I am He” is actually “I am” in the Greek, without the “He” and undoubtedly contains overtones of deity. The verse should read, “I am telling you now before it happens, so that when it does happen you will believe that I am.” Jesus is looking for a full and complete faith in His disciples as a result of what is about to happen to Him.
See also the following verses:
Isaiah 48:5 Therefore I told you these things long ago; before they happened I announced them to you so that you could not say, ‘My idols did them; my wooden image and metal god ordained them.’
John 14:29 I have told you now before it happens, so that when it does happen you will believe.
John 16:4 I have told you this, so that when the time comes you will remember that I warned you. I did not tell you this at first because I was with you.