Is a Christian who teaches error still a brother in Christ?

Answer:

The question boils down to whether a brother who teaches a false doctrine is still a Christian. Why this one particular sin is being focused on, I don't understand. All sins are alike to God.

It looks as if the main problem is semantics. He is using "brother" to mean in fellowship. You are using "brother" to mean he is in the covenant with Christ and is a Christian.

Matthew 12:50 and Mark 3:35 teaches that those who do the will of the Father are in fellowship with Christ. It doesn't address the question of whether someone, who once was in fellowship and departed, is a Christian. I do agree that he is no longer in fellowship with God and Christ. "If we say that we have fellowship with Him, and walk in darkness, we lie and do not practice the truth" (I John 1:6). Without that fellowship with God, the sinning Christian would no longer have fellowship with faithful Christians. But that brings us back to the question: Does he cease to be a Christian?

He referred to: "But now I have written to you not to keep company with anyone named a brother, who is sexually immoral, or covetous, or an idolater, or a reviler, or a drunkard, or an extortioner--not even to eat with such a person" (I Corinthians 5:11). This verse does emphasize that there cannot be fellowship with a Christian who is in sin, but it actually supports your point as well: the sinning Christian is still a Christian. Why? Because we cannot keep company with brethren who are in sin, we still have to keep company with people who have never entered into Christ. "Yet I certainly did not mean with the sexually immoral people of this world, or with the covetous, or extortioners, or idolaters, since then you would need to go out of the world" (I Corinthians 5:10). The sinning Christian is treated in a different manner from worldly people, even though he has returned to living by worldly standards.

The verse you brought up makes the same point: "And if anyone does not obey our word in this epistle, note that person and do not keep company with him, that he may be ashamed. Yet do not count him as an enemy, but admonish him as a brother" (II Thessalonians 3:14-15). A fallen brother is still a brother and is to be treated with some respect even while he is being rebuked for his sins and fellowship is being withdrawn from him.

What, then, do we have? We enter into the fellowship of Christ by doing the will of God through faith. That gives us the right to be called brothers. "For you are all sons of God through faith in Christ Jesus. For all of you who were baptized into Christ have clothed yourselves with Christ. There is neither Jew nor Greek, there is neither slave nor free man, there is neither male nor female; for you are all one in Christ Jesus" (Galatians 3:26-28). We enter into a covenant with God. "And in Him you were also circumcised with a circumcision made without hands, in the removal of the body of the flesh by the circumcision of Christ; having been buried with Him in baptism, in which you were also raised up with Him through faith in the working of God, who raised Him from the dead" (Colossians 2:11-12).

That makes an interesting point. An Israelite became a part of the covenant when he was circumcised. He might stumble, he might leave God, but he would still be in the covenant. In fact, it is because he is in the covenant that God works to restore him (Ezekiel 18:21-32). If he is restored, he did not have to be circumcised all over again because he was still in the covenant.

In the same way, a Christian who falls away can be restored, but he doesn't need to be baptized again because he is still in the covenant with Christ. The reason we work at trying to restore a fallen brother is that it is required by the covenant. It is because he is a fallen brother we must withdraw our fellowship with him while he remains in sin. Being a Christian is not a guarantee that you will be saved. Having become a brother in God's family does not guarantee you will enter heaven.