R
True Christians are identified by abiding (1 John 2:14, 4:15; 2 John 1:2 etc..). And if someone doesn't, if they go out, reject, deny, they went out because they were not of us, if they had been of us they would have remained with us. (1 John 2:19).
Remaining, staying, abiding is the mark of true salvation. Some people turn "abiding in Christ" into some upper level spiritual experience that only super saints do. It's not that. It means to stay or remain in the truth, in Christ. Verse 24, "As for you, let that remain in you which you heard from the beginning...if what you heard from the beginning remains in you, you will also remain in the Son and the Father."
Remaining, staying, abiding is the mark of true salvation. Some people turn "abiding in Christ" into some upper level spiritual experience that only super saints do. It's not that. It means to stay or remain in the truth, in Christ. Verse 24, "As for you, let that remain in you which you heard from the beginning...if what you heard from the beginning remains in you, you will also remain in the Son and the Father."
19They went out from us, but they did not really belong to us. For if they had belonged to us, they would have remained with us; but their going showed that none of them belonged to us.
20But you have an anointing from the Holy One, and all of you know the truth.e 21I do not write to you because you do not know the truth, but because you do know it and because no lie comes from the truth. 22Who is the liar? It is whoever denies that Jesus is the Christ. Such a person is the antichrist—denying the Father and the Son. 23No one who denies the Son has the Father; whoever acknowledges the Son has the Father also.
24As for you, see that what you have heard from the beginning remains in you. If it does, you also will remain in the Son and in the Father. 25And this is what he promised us—eternal life.
26I am writing these things to you about those who are trying to lead you astray. 27As for you, the anointing you received from him remains in you, and you do not need anyone to teach you. But as his anointing teaches you about all things and as that anointing is real, not counterfeit—just as it has taught you, remain in him.
28And now, dear children, continue in him, so that when he appears we may be confident and unashamed before him at his coming.
A simple honest read shows us John is not at all suggesting only fake believers who never believed to begin with leave. He's plainly warning those who have an anointing and who know the truth to not leave like these others, who didn't know the truth, did.