Do not be unequally yoked with unbelievers. For what partnership has righteousness with lawlessness? Or what fellowship has light with darkness? What accord has Christ with Belial? Or what portion does a believer share with an unbeliever? What agreement has the temple of God with idols? For we are the temple of the living God; as God said, I will make my dwelling among them and walk among them, and I will be their God, and they shall be my people. -- 2 Corinthians 6:14-16
The caution also extends to common conversation. We should not join in friendship and acquaintance with wicked men and unbelievers. Though we cannot wholly avoid seeing and hearing, and being with such, yet we should never choose them for friends. We must not defile ourselves by converse with those who defile themselves with sin.