This saying is true. Therefore rebuke them sharply, so that they will be sound in the faith
NMVThis witness is true. For what reason rebuke (reprimand; strongly warn; restrain) them sharply, that they may be sound in the faith;
OJBThis eidus (testimony) is true, for which cause rebuke them sharply, that they may be orthodox Jewish in emunah,
T4TThe words that this man said about the Cretan people are still true, especially about their being liars. So, rigorously convince/show those Cretan believers that those false teachings are wrong. Tell them to firmly believe in the correct teachings.