Albert Barnes Commentary John 13:19

"From henceforth I tell you before it come to pass, that, when it is come to pass, ye may believe that I am [he]." — John 13:19 (ASV)

Now I tell you before it come, and so on. They would see by that that he had a knowledge of the heart and the power of foretelling future events, and must therefore have been sent by God. This does not imply that they had no faith before this, but that their faith would be increased and strengthened by it.