I have been studying your sermon outline "Withdraw Yourselves." I have a friend who was once a member of the Lord's church. He is now a member of a denomination. Your outline says that you cannot withdraw from a non-Christian. II John 10-11 says, "If anyone comes to you and does not bring this doctrine, do not receive him into your house nor greet him; for he who greets him shares in his evil deeds." It is obvious that denominations teach false doctrines. Does this mean that I am sinning if I enjoy vacations, meals, etc. with his family?
"I wrote to you in my epistle not to keep company with sexually immoral people. 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. 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 5:9-11).
We tend to socialize with people with whom we feel we have a common bond. The problem comes when someone is a Christian, but no longer practices the truth. If we continue our association with him we are telling him and the rest of the world that we find nothing disagreeable about the person.
Though this man is now attending a denomination, you still acknowledge that he had been a pure Christian in his past. "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). He is no longer obeying the commands of the apostles, so we are not to socialize with him to make our disagreement plain.
When you keep socializing with the man, you are encouraging him to remain as he is. Encouraging someone to remain in sin is also a sin.