A.T. Robertson 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)

From henceforth (απ' αρτ). "From now on," as in 14:7; Matthew 23:39; Revelation 14:13.

Before it come to pass (προ του γενεσθα). Προ with ablative of the articular second aorist middle infinitive γινομα (before the coming to pass).

When it is come to pass (οταν γενητα). Indefinite relative clause with οταν and the second aorist middle subjunctive of γινομα, "whenever it does come to pass."

That ye may believe (ινα πιστευητε). Purpose clause with ινα and present active subjunctive of πιστευω, "that ye may keep on believing." Cf. Isaiah 48:5.

That I am he (οτ εγω ειμ). As Jesus has repeatedly claimed to be the Messiah (8:24,58, etc.). Cf. also 14:29 (πιστευσητε here); 16:4.