One of the things that I find many Christians don't understand is discipline. The Bible says in Romans 11:22 that God is kind and severe. May believers know him as kind. They don't understand his severity at all, and so they have an imbalanced view of God. It's like a half a body. It's not a complete body. If you don't understand the severity of God—you only understand his kindness—you don't understand God at all. If you understand only his severity and you don't understand his kindness, you don't understand God at all.
God is strict, and he's good, llke a father. A father cares for his children, provides for his children, but disciplines them. Jesus taught about discipline in the church in Matthew 18, verse 15 onwards. "If your brother sins, go and speak to him alone, and if he listens to you, you have won your brother." If he does not listen to you, what should you do? Take one or two brothers with you that every fact may be confirmed, and if he still does not listen to you, go and tell it to the church—that is, the elders. If he doesn't listen to the elders in the church, "let him be to you as an unbeliever." So, if a person has rejected the last step and he doesn't even listen to the elders, you have to treat him like an unbeliever.
Now, you may try to act kinder than God and be stupid. There are many believers like that. They try to act more kind than Jesus and they're absolutely one hundred percent stupid. They hinder God's work. But, I obey God's word. If he doesn't listen to the church, I'll treat him like an unbeliever. That doesn't mean I hate him. I'm not supposed to hate unbelievers. That doesn't mean I don't forgive him. I'm not supposed to be unforgiving. Jesus forgave unbelievers on the on the cross. I forgive him, I love him, but I treat him like an unbeliever. I don't have fellowship with unbelievers. I can't have fellowship with him.
If he repents, I'm not saying he's got to come back to our church. If he's repented of his sin and there's evidence of it, we can have fellowship with him. But, if there's no evidence of that, you are hindering God's work in his life by fellowshipping with him.