'Unbeliever' in the Bible

Yet if the unbeliever departs, let there be separation. The brother or the sister is not under bondage in such cases, but God has called us in peace.

What agreement has Christ with Belial? Or what portion has a believer with an unbeliever?

But if anyone doesn't provide for his own, and especially his own household, he has denied the faith, and is worse than an unbeliever.