2 corinthians 6:14-18

4Do not be yoked together with unbelievers. For what do righteousness and wickedness have in common? Or what fellowship can light have with darkness? 15What harmony is there between Christ and Belial^[a]? What does a believer have in common with an unbeliever? 16What agreement is there between the temple of God and idols? For we are the temple of the living God. As God has said: “I will live with them and walk among them, and I will be their God, and they will be my people.”^[b]
17“Therefore come out from them
and be separate, says the Lord.
Touch no unclean thing,
and I will receive you.”^[c]

“don’t team up with those who are unbelievers” it says it straight out. More in depth, A non believer is held to a different set of laws than we are. They are held to laws of evil and its strongholds. They don’t have the spirit in them to help give them strength and they don’t realize what is wrong unless they know God and his laws. If somebody does happen to date a non-believer, they are disobeying God’s law. If we break one part of the law, we break it all. (james 2:8-11) There for, it is just as bad for him to do that, as it is for him to fall back into his old life while killing babies. It is a sin to date a non believer.