1 John 2:19-20

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. But you have an anointing from the Holy One [you have been set apart, specially gifted and prepared by the Holy Spirit], and all of you know [the truth because He teaches us, illuminates our minds, and guards us from error].