1 John 2:19

They went out from us Separated themselves from the communion of the true church of Christ. Hence it is one of the marks of antichrist, that he had been once in the Christian Church, and a teacher by profession, but had left it or apostatized; but they were not of us When they went, their hearts were before departed from God; for if they had been of us Had been inspired by the same spirit wherewith we are inspired; they would, no doubt, have continued with us For upright men, of a pure intention, would never have seen any cause to leave us, and divine grace would have preserved such from being overcome by the temptations of these artful deceivers; but they went out They were permitted to apostatize outwardly; that they might be made manifest See 1 Corinthians 11:19. (This was made manifest by their going out;) that they were not all of us Sound members of our body, really believing the same truths which we believe, and partaking of the same grace which we partake of.