Jude 1:5

Though ye once knew this,— Though ye fully, or perfectly. See Jude 1:3. The Christians to whom St. Jude writes, had formerly been of the Jewish religion, and were therefore well acquainted with the Old Testament, from their hearing it read in the synagogue every Sabbath-day. It is intimated in the latter clause, that the grand corrupters of the gospel referred to, were guilty of unbelief or disobedience to God; in which if they persisted, all their Christian privileges would not prevent their destruction.