Over the years I've had the opportunity (as a youth leader, as a pastor, as a dude) of reaching out to atheist teenagers whom I've encountered in various situations (at youth group, in school, on the street.) During these encounters I've developed my own set of relational "rules of engagement" that I follow when reaching out to teens who don't believe in God. I've discovered that, if I want to have any real chance of reaching them with the gospel, the closer I stick to these the better. Here they are:
1. Mock religion as early as you can in the conversation.
The Latin word for religion comes from verb that means "to bind or link back." It is man's attempt to link or bind himself back to God through good deeds and/or religious rituals. But in Isaiah 64:6 God calls these attempts "filthy rags" or, in the words of Propaganda in the Life in 6 Words video, "even your good deeds are an extension of your self-righteousness."
When atheist teenagers discover that God is as disgusted by religion as they are it gives them common ground with authentic Christianity. After all it was religious people that Jesus railed against in Matthew 23. It was religious people who crucified Jesus in Mark 15. It was religious people who killed Stephen, the first martyr of the early church, in Acts 8.
Gain this common ground as soon as possible with atheist teenagers. When they see you sickened by the hypocrisy that inevitably accompanies religion, the emotional barriers that often keep them from taking a second look at Christianity can begin to fall down.
2. Focus on Jesus.
Jesus was a radical, rebel and revolutionary. This same "vibe" often appeals to atheist teenagers, many of whom consider themselves the same.
Show them stories in the Bible where Jesus healed lepers, hung out with "sinners" and bucked the religious system. Paint the picture of Jesus as a hero of the downtrodden (because he was) and his death as the ultimate injustice (because it was!) When they begin to see Jesus' willingness to suffer injustice so that they could be justified the code of unbelief can be cracked in their souls.
3. Speak of God as if he exists.
Instead of assuming they are true atheists, speak of God as a reality. Romans 1:18-21 makes it clear that,
"The wrath of God is being revealed from heaven against all the godlessness and wickedness of men who suppress the truth by their wickedness, since what may be known about God is plain to them, because God has made it plain to them. For since the creation of the world Godâ€™s invisible qualitiesâ€”his eternal power and divine natureâ€”have been clearly seen, being understood from what has been made, so that men are without excuse. For although they knew God, they neither glorified him as God nor gave thanks to him, but their thinking became futile and their foolish hearts were darkened."
At the end of the day there are no true atheists. In the deepest parts of their soul every atheist, according to Romans 1, truly believes in the existence of God but doesn't want to give glorify him or give him thanks.
I have found that the more you speak of God as a reality the sooner the atheist teen will say something like, "Well, if God is so good then why...." It's at this moment they are admitting that they actually do believe in God but they don't want to acknowledge it for whatever reason. So, instead of attacking their arguments, speak of God as if he exists and let them deal with the true core of their problem with God, not his existence but their accountability to him.
4. Pray, love, repeat.
Keep praying for these teenagers in spite of their protests against God. Soon, they like Saul, may find themselves kicking against the goads of unbelief. Soon, they like Paul, may find themselves with a new identity in Christ. And love them no matter how they respond to Jesus, even if they never respond to Jesus. But this brand of 1 Corinthians 13 "love from above" will be hard for them to resist.