2 Corinthians 6:14-18

 14Do 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? 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.”
 17”Therefore come out from them and be separate, says the Lord.
   Touch no unclean thing, and I will receive you.”
 18”I will be a Father to you, and you will be my sons and daughters, says the Lord Almighty.”

MEDITATION
Christians are to be completely separated unto the Lord, with that separation
as sharp as the dividing line between light and darkness, between Christ and
Satan, and between the temple of God and a shrine where idols are worshiped.

In short, we are to “purify ourselves from everything that contaminates body
and spirit.” (7:1) God is our Father, but when we sin, sin’s contamination
separates us from God.  He is not able to be a Father to us in the same,
intimate way He would if we were in close fellowship with Him.

Similarly, when sin comes into a relationship between two people, it’ll be felt like a
grain of sand entering into the eye, the relationship will be affected.  So, the
issue isn’t whether I could have intimate relationship with non-Christians,
rather, if I want to maintain this close relationship with God the Father.  Our
loving father is calling us: “I will be your father, and you shall be my sons and
daughters.”  How do I respond?

Personal Note:
This passage is used as a basis for not having intimate relationship with non-believers.  This is true, but there is a bigger context than this.  We are to separate ourselves from the things of this world that causes us to sins.  Are the friends you are hanging out with causing you to stumble?   If so, for the sake of developing a close relationship with God, it would be wise to be separated from them.

Hey, don't we teach about reaching out and sharing the Gospel to non-believers in the church?  Please note the difference between friends we reach out to and friends we yoke with.  Friends we reach out to are guided by God through prayer, which friends we yoke with are the ones who may influence us.