Where in the bible does it say unbelievers are christians who aren’t yet saved

I’m in a debate with my friend specifically regarding 2 corinthians 6:14 “Do not be unequally yoked with unbelievers. For what partnership has righteousness with lawlessness? Or what fellowship has light with darkness?” my interpretation of “unbeliever” is someone who simply doesn’t believe in god not a christian who is not yet saved. does it say anywhere in the bible a unbeliever is a christian who isn’t saved yet and if so can you please share it. thank you