Jesse Johnson of The Cripplegate discusses whether or not believers should have non-believers as close friends. Watch “Ask A Prof.”
Well there are two biblical concepts at play in this question—the first is the principle of purity and the second is the mandate for evangelism.
The Principle of Purity:
Christians are called to be pure. We are supposed to guard our hearts, our thoughts, and our ways. The danger of a friendship with a non-believer is that might bring the Christian down. It could bring a Christian down morally, intellectually, and spiritually.
The Bible says that “Bad company corrupts good morals” (1 Corinthians 15:33). Be spending time around those who don’t love Jesus or fear the Lord, we run the risk of negatively impacting our own morality, hampering our world-view, and compromising spiritually.
That is not a prohibition to Christians against these kind of friendships; but it is a warning—a warning that our friendships we guard our hearts, minds, and affections.