What your boyfriend is actually saying is that he doesn’t think he needs God — and that should concern you very much. Not only does it create a barrier between you right now, but it should warn you about potential problems in the future — especially if you were to get married. The Bible warns, “Do not be yoked together with unbelievers” (2 Corinthians 6:14).
What can you say to your boyfriend? First, you might ask him how he defines sin. Is it only a horrible crime or act of violence? Is it only being enslaved by a terrible habit, like drugs or alcohol? If that’s all sin is, then he’d probably be right in his position. He reminds me of the man in one of Jesus’ stories who declared, “God, I thank you that I am not like other people — robbers, evildoers, adulterers” (Luke 18:11).
But your boyfriend (like the one in Jesus’ story) is guilty of a sin that’s even greater than any of these. It is the sin of pride, and the reason it’s so serious is because it cuts us off from God. We think we don’t need God, and we don’t want anything to do with him.
Pray for your boyfriend, that God will open his eyes to his need for Christ. Ask God also to help you be an example to him of God’s love and purity and righteousness. Your friend needs Christ, and God wants to use you to help him face his need.