1 Jn.2:19 "they went out from us but they were not of us, if they had been of us , they would have continued with us, but they went out that they might be made known that none of them were of us." John is writing about those who denied Jesus came in the flesh (meaning they did not believe God came as a human and rose bodily). They left the fellowship of believers that were holding to the apostles doctrine. Today many other aberrations just as false have arisen, and given birth to offshoots in the Church.
