1 John 2:18-19

Children, it is the last hour [the end of this age]; and just as you heard that the antichrist is coming [the one who will oppose Christ and attempt to replace Him], even now many antichrists (false teachers) have appeared, which confirms our belief that it is the last hour. They went out from us [seeming at first to be Christians], but they were not really of us [because they were not truly born again and spiritually transformed]; for if they had been of us, they would have remained with us; but they went out [teaching false doctrine], so that it would be clearly shown that none of them are of us.