Christian fellowship does not extend outside or beyond the Church. In fact, to do so is sin. This is not to suggest that Christians must withdraw from the world because we have to engage the world if we are to seek and save it. Jesus ate with sinners so that He might reach them. So while we must “associate” with the world, we do not fellowship it (1 Cor. 5:9,10). In 2 Corinthians 6:14, Christians are warned “do not be bound together with unbelievers; for what partnership have righteousness and lawlessness, or what fellowship has light with darkness?” Christians have to discriminate in choosing who to fellowship. He chooses not to have fellowship with the dark and sinful things of Satan. Christians shouldn’t fellowship a Brother who is openly living in sin. Paul writes, “remove the wicked man from among yourselves” and you are “not to eat with such a one” (1 Cor. 5:13). Christians do not fellowship false teachers or those who refuse to obey the Gospel. Often the lines must be drawn over matters of doctrine. Buster Dobbs writes,
If a person rejects the teaching of the Bible on any given subject, he has refused truth and embraced error. He has denied unity and promoted division. Those who deny baptism involves immersion and resurrection are guilty of dividing. Teachers who say that baptism is not “for” the remission of sins deny revelation and, thereby, deny Jesus. People who accept mechanical instruments in worship of God, instead of offering a sacrifice of harmonious human voices, are disturbers of peace — it is sin. Teachers who deny the verbal inspiration of the sacred Scriptures rend the body of Christ. The list goes on and on and includes every aspect of the nature, purpose, and work of the church for which Jesus died. (link to article)
The line of fellowship must be drawn on such issues for the sake of unity. For this reason, Paul said to “reject a factious man” and Jesus said, “shake the dust off your feet” and move on to teach others (Mt. 10:14). The Christian makes a distinction between good and evil; between Christian and non-Christian: he fellowships one and not the other. It’s not easy or comfortable, but it is necessary.
The sharing or joint participation of the Christian community is called fellowship. It is seen in the sharing of worship and in the regular contribution (1 Cor. 10:16). The word koinonia occurs 17 times in the New Testament. In Acts 2:42, the Christians “were continually devoting themselves to the apostles’ teaching and to fellowship,…” The same way they were devoted to prayer and worship, they were devoted to sharing their resources and time with one another. “And all that believed were together, and had all things in common” (Acts 2:43). That’s fellowship and it’s beautiful. In 1 Corinthians 1:9, Paul says “our fellowship is with His Son, Jesus Christ our Lord.” This is the Christian’s most important relationship. The Christian walks with Christ in close communion as in an intimate friendship, and it is unlike anything Christians have with the world. John writes that we have fellowship with and forgivness from the Father as we walk in the light (1 Jn. 1:6,7). When we walk in faith and confess our sins, God shares his grace “because the blood of Jesus His Son cleanses us from all sin” (1 John 1:6,7). The Lord is the Light and with Him Christians fellowship.
Now, the question before every Believer is who are you fellowshiping? It might be a valid question to ask if we are to fellowship only Christ and His Church, are we not also to share and do good to all men? The answer is yes, we are to do good to all men, but Paul says we give preference to “the household of the faith” (Gal. 6:10). Fellowship is the reason such a distinction is made. God wants His family together, both helping, serving and reaching out to the lost. But the church family should be especially devoted to the fellowship of like-believing people.