Wherefore come out from among them, and be ye separate, saith the Lord, 2 Corinthians 6:17a
The Word of God tells us that we should be separated from the world. We shouldn't look like them, act like them, or talk like them. We should be distinguishable from them.
In Verse 14 of the same chapter, we are told: Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness? We aren't to marry an unbeliever, but also we shouldn't be hanging out with them either. Remember your parents telling you "you are who your friends are"? If you're hanging around unbelievers, you'll look, act, and talk like them - AND no one will be able to recognize you're not one of them.
This separation is also known as Sanctification.