Charles Ellicott 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.—The marginal rendering is to be preferred. (John 14:7.)

Ye may believe that I am he.—Compare to Note on John 8:24; John 14:29. The result of His declaring these things to them from that time forward, before the events, will be that they will find confirmation of their faith in Him as the Messiah. Had He not then declared His knowledge of all, and traced even His choice of Judas to the will of God, there would have been room for doubt whether that choice was consistent with His being the Messiah.