Christians have to constantly face temptations and the attacks of the world around us. Everything we see, read, do, put in our bodies, etc. affects us somehow. That's why to maintain a close relationship with God, we have to put aside our old ways of doing things; the things we watch on TV, old bad habits (excessive drinking, smoking, etc.), the activities we participate in, and the people we spend our time with. People are divided really only two ways; belonging to the world (Satan), or belonging to God (Acts 26:18).
The book of Proverbs has a few wise verses on believers befriending non-believers: "The righteous should choose his friends carefully, for the way of the wicked leads them astray" (v12:26). We should stay away from foolish people (vs. 13:20, 14:7) and people who lose their temper easily (22:24), and keep away from the rebellious (24:21). All these things represent those who have not been saved. "Do not be yoked together with unbelievers. For what do righteousness and wickedness have in common? Or what fellowship can light have with darkness" (2Cor 6:14)? 1Corinthians 15:33 tells us that bad company corrupts good character. Unbelievers are slaves to sin (John 8:34), and Christians are slaves to God (1Cor 7:22). If we become deeply involved (either by friendship or a romantic relationship) as Christians with non-Christians, we are setting ourselves up for turmoil. It can (and does often) cause the Christian to stumble in their walk, fall back into a sinful life, and also turn others away from God (by misrepresenting God and Christianity).
Although these close relationships are not recommended, it does not mean we turn our noses up and ignore unbelievers either. 2 Timothy 2:24-26 tells us that as servants of the Lord, we are to be kind to and not quarrel with anyone. We should gently teach those who oppose the truth, and be patient with difficult people. Matthew 5:16 tells us “...let your good deeds shine out for all to see, so that everyone will praise your heavenly father.” We should serve unbelievers so that they may see God through us and turn to Him in praise. James 5:16 says that there is great power in the prayer of a righteous person, so bring your concerns for unbelievers before God and He will listen.
Many people have been saved because of the prayers and service of Christians, so don't turn your back on unbelievers. Having any kind of intimate relationship with an unbeliever can quickly and easily turn into something that is a hindrance to your walk with Christ. We are called to evangelize the lost, not be intimate with them. There is nothing wrong with building quality friendships with unbelievers – but the primary focus of such a relationship should be to win them to Christ, by sharing the Gospel with them and demonstrating salvation in your life.
http://www.gotquestions.org/friendships ... evers.html