We're going through 1 John as a family right now, and what is interesting about John is that he roots everything in the two greatest commandments, i.e., love for God and fellow believers. He argues that one who loves God will love his brother, and that the one who is apathetic toward a fellow Christian doesn't walk in the light (i.e., illumination that comes from the truth), but remains in the darkness to this very moment. Anyone who says he knows Christ but doesn't care for a fellow Christian is a liar and the truth is not in him. After this, he writes that this message is for every Christian of all ages (e.g., little children, younger men, older men).
But in the midst of all of this talk about love, John suddenly turns to a discussion concerning the doctrine of Christ. He states that the antichrist is coming and that many antichrists have come into the world already, and that those who follow a false Jesus are "not of us," and never "were of us." He then encourages his fellow believers to remain in the truth they heard originally concerning Jesus, that He is the Christ and the Son. It is the one who believes in the Father and the Son, not just a Jesus who is merely human or not human at all, who have the right Jesus who have truly come to the light.
What is interesting about this is that many would think that John is no longer talking about loving God by listening to what He instructed and loving one's fellow Christian, but then John continues right on discussing that very thing.
This brings me to conclude something that has become lost on our generation that sees theological discussion as something that is merely intellectual or less important than being loving. accepting, or nice to other people. And that is this, that correcting doctrine is loving God and others and not correcting it is hatred toward God and others.
Why? Because you wouldn't be loving your wife by confusing her with some other woman. You wouldn't be loving your children by lying to them and telling them that some other woman was their mother. By doing both of these, you ignore and abandon your real wife and cause your children to do the same.
But what is worse is that the real Jesus is the only One who can accomplish for others what John lays out in the beginning of the epistle, which is a cleansing of all unrighteousness and a forgiving of all sins. Only the Logos of God who has come in the flesh, who the apostles saw with their own eyes and touched with their own hands, and heard from the beginning, can save people. Hence, to teach another Jesus (i.e., an antichristos "replacement Christ") is to abandon the real Jesus in dishonor and to damn others rather than save them.
John further states that to do this is to not really worship God either. Instead, the person has a false god who is not really the Father, as he says that only those who have the Son have the Father also. The one who denies the Divine Son also denies the Father. Hence, to reject the real Jesus is to reject God Himself. Sound like loving God to you?
What I think John is getting at here by including this discussion in the midst of talking about love is that doctrine isn't just some heady enterprise where we discuss lofty philosophical ideas that don't really have anything to do with the core of real Christianity. Instead, correct doctrine is a matter of love, loving God and loving one another.
Hence, it would be unloving of John to include those who teach or adhere to false doctrines concerning God and Jesus as a part of the group. They are "not of us" nor have they ever been "of us." He tells this to Christians that they might know that doctrine matters and only those who have been anointed by the Holy One seem to get that.
Correcting doctrine, then, is an act of love, even if the person correcting you doesn't have fuzzy feelings for you. If you asked me if I would rather have you feel a certain way about me or save my life, I think I'd choose the latter.
If you love God, the real God who exists, let Him be honored and exalted by connecting people to Him and not some other god instead. If you love Jesus, let Him be honored and exalted by bringing others to Him, and not to some other Jesus who has been placed in His stead. And if you love people, put them on the right path by teaching them about the true God and Jesus so that they can not only glorify the Father and Son in recognition and worship of them, but be forgiven and made clean.
And remember, correct doctrine is a matter of love for all who walk in the light. False doctrine is a matter of hatred toward God and man. It's time that we all start seeing it as such.