Should Christians be friends with non-believers?
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.
The Mandate for Evangelism:
To counter the principle of purity, we remember that we also have a mandate for evangelism. We have received the great commission, and respond knowing that we are to go into all the world to make converts from non-believers. And one of the most effective ways to evangelize is through our friendships.
When we befriend a non-Christian, we can use our friendship to open up conversations about the gospel. But these kind of friendships are not token, and they cannot be superficial. They have to based on real love for the person, and a real sense of common interests and affections, with a real concern for the well-being of the other. In other words, it has to be an authentic friendship, not a manipulated one.
The purpose behind these kind of friendships is the love for the other person—you want what is best for him—so it must make it back to the gospel. The warning for friendship evangelism is that his has to come back to the gospel, or it represents a missed opportunity. You have to ask the questions that take the friendship from the things of this world into the things of eternity.
So should Christians be friends with unbelievers? Absolutely, if they are using their friendships to advance the gospel of our Lord Jesus Christ.