Paul heard a Judaizer say "Cretans are always Liars." The source is a Cretan philosopher named Epimenides. Showing the distinction of contradiction... I think Paul was a learned enough man to expose the false prophets, for making a show of knowledge that the statement is paradoxical... for the Judaizer told the truth, but in that truth were indeed a veiled lie. Showing "Cretans are always liars" is itself a lie, as that syntactic idea exposes the unbeliever and the Judaizer, that even when telling the truth, they still lie.