John Calvin 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)

I tell you this now, before it happen. By this statement he reminds his disciples that, when one of their number becomes a reprobate, this is so far from being a good reason for their being discouraged, that it ought to be a fuller confirmation of their faith. For if we did not see before our eyes, in the Church, what has been foretold about her distresses and struggles, a doubt might justly arise in our minds: Where are the prophecies? But when the truth of Scripture agrees with our daily experience, then we perceive more clearly that God takes care of us and that we are governed by his providence.

That you may believe that I am. By the phrase, that I am, he means that he is that Messiah who had been promised; not that the conduct of Judas as a traitor was the first event that led the disciples to exercise faith, but because their faith made greater progress when they experienced those things which they had previously heard from the mouth of Christ. Now, this may be explained in two ways: either that Christ says that they will believe after the event has happened, because nothing was hidden from him, or that nothing will be lacking in him of all that Scripture testifies concerning Christ. Since the two interpretations agree well enough, I leave my readers free to choose which of them they prefer.