4. GOD DOES NOT FELLOWSHIP CHRISTIANS WHO ARE IMPENITENTLY PREACHING ERROR.
Being a baptized, penitent believer is necessary but not sufficient. The Bible does not teach the notion of "once saved, always saved." One must remain faithful if he wants to maintain fellowship with God. Our Lord taught in Matthew 10:22 - "And you will be hated by all for My name's sake. But he who endures to the end will be saved." Those who uncompromisingly teach the truth--even in a spirit of love--will make plenty of enemies (both in and out of the church). But, though the world hate such a one, God will save him who is faithful to the end! Such a one will receive a glorious crown of life (cf. Rev. 2:10)! Being a baptized, penitent believer puts one into God's spiritual light and cleanses him from all sin, but he then must continue walking in the light. If he starts drifting from the truth (cf. Heb. 2:1), God will try to bring him back through His word and through the rebukes of the faithful. But, eventually God will give up on him if he refuses to repent. He will turn such a one over to the darkness he has chosen; He will sever His fellowship.
This happened to some in the 1st century church - "You have become estranged from Christ, you who attempt to be justified by law; you have fallen from grace" (Gal. 5:4). Notice that they were estranged ("severed" - ASV) from Christ regarding their fellowship with Him. How did this happen? Because some of them were making laws where God had not made them. Some were teaching that Christians must keep the Old Testament--even the practice of circumcision--in order to be saved. The apostles had already dealt with this matter in Acts 15, but some didn't listen. God no longer required the keeping of the Mosaic law; it was no longer a binding law system but merely a document to learn from (cf. Rom. 15:4). God was patient with these Christians for a while, but when they refused to let go of their heretical teachings (even binding them on others), they fell from His grace (i.e., they left His light). To be entangled with the Old Law and to require others to keep it was (and is) false doctrine. Those who continued to preach this error had fallen out of God's favor. It does matter what we preach and teach! Yes, these were baptized believers, but they weren't living penitently anymore! They were involved in sin (specifically, teaching false doctrine) and until they turned from it, God had severed His fellowship with them. When a Christian teaches false doctrine today and will not turn from it, eventually God vomits him out of His mouth (cf. Rev. 3:15-17).
5. GOD DOES NOT FELLOWSHIP CHRISTIANS WHO ARE IMPENITENTLY PRACTICING ERROR.
This truth is closely related to the prior point and can be illustrated from several places in the New Testament. In I Corinthians 5, for example, a Christian man was involved in an immoral sexual relationship. He was living in sin; he was practicing error. The church at Corinth was knowledgeable of the situation, but they hadn't taken proper action! Woe to congregations today (elders particularly) who do likewise, turning a blind eye to sin they know is present in the body. When cancer is positively identified in one's body, it is foolish to ignore it and consequently encourage it to spread further. Yet that is what many cowards in the church do regarding spiritual cancers. Jesus set forth an important and appropriate teaching on this matter - "Moreover if your brother sins against you, go and tell him his fault between you and him alone. If he hears you, you have gained your brother. But if he will not hear, take with you one or two more, that by the mouth of two or three witnesses every word may be established. And if he refuses to hear them, tell it to the church. But if he refuses even to hear the church, let him be to you like a heathen and a tax collector" (Matt. 18:15-17). The man in Corinth was already out of fellowship with God because of his sinful behavior of which he had not repented. But, the church wasn't recognizing the line that God had drawn! They were still sharing and participating with him, acting as if nothing was wrong! There is no evidence that they had corrected him individually or as a congregation. God commanded that they withdraw from him that he might come to his senses and repent "that his spirit may be saved in the day of the Lord Jesus" (I Cor. 5:5). Even when God disciplines, He shows His love.
The church has a responsibility to keep its membership pure, as much as possible. If a Christian is sinning and others know about it, then they must confront the erring brother in a spirit of gentleness in hopes of convincing him of his need to repent and be restored (cf. Gal. 6:1). However, if a brother will not repent, then God will withdraw fellowship from him, and the church must do likewise. Regarding discipline in the church, whatever is bound on Earth has already been bound in heaven and whatever is loosed on Earth has already been loosed in heaven (cf. Matt. 16:19; 18:18). We must respect the lines God has already drawn!
Let us briefly read Paul's instructions regarding withdrawing fellowship in II Thessalonians 3:6,14,15 - "But we command you, brethren, in the name of our Lord Jesus Christ, that you withdraw from every brother who walks disorderly and not according to the tradition which he received from us...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." The church cannot fellowship anyone who is living in sin and will not repent (cf. Eph. 5:11). Again, we must respect the lines God has drawn.
Dear friends, I hope you've seen that whom we fellowship is important because it is important to God! Ultimately, we do not determine whom we should or should not fellowship. God has already done that, and we must respect His will. God fellowships baptized, penitent believers--and that is whom we must fellowship. Whether they are strong or weak, we must fellowship them. Even if they hold different opinions than us on matters of indifference, we must fellowship them. Even if they're drifting spiritually, we must fellowship them and encourage them to be faithful. The fellowship between Christians walking in God's light is a tremendous blessing and full of wonderful potential. Spiritual fellowship should not be treated casually but rather cultivated.
We all know that God does not fellowship Satan and He doesn't fellowship those who are living immorally. Though many are reluctant to acknowledge it, He also doesn't fellowship those who are practicing false religions (whether they claim to be "Christian" or not). He doesn't continue fellowshiping those who once were in the light but have re-entered the darkness by impenitently preaching or practicing error. God doesn't fellowship any of these individuals, and neither should we!