There is no single act in the Christian life that will trigger spiritual maturity in the life of a teenager more quickly and completely than evangelism. When teenagers open their mouths to proclaim the fame and Name of Christ, especially to their friends, they are risking something they hold most dear… relationships.
Ask any teenager what means most to them on this earth and most of the time the answer will come back centering around their friendships, relationships and/or how they are viewed by their peers. Evangelism risks all of these to follow the command of Christ and, as a result, triggers exponential spiritual growth in the heart of the evangelizing teenager.
When Abraham was willing to offer his son Isaac on the altar this was a courageous act that resulted in exponential spiritual growth in his own heart. According to James 2:22, "his faith and actions were working together and his faith was made complete by what he did." In the original Greek language the words "made complete" could literally be interpreted "matured." In other words, Abraham's willingness to sacrifice what meant most to him resulted in accelerated spiritual maturity in Abraham.
What's true of Abraham is true of Christian teenagers. When teenagers are willing to sacrifice their friendships if need be so that their friends can hear the message of Jesus it results in exponential spiritual growth. They pray more (because they are afraid.) They depend on God's Spirit more (because they need His power!) And they study the Bible more (because they want to know how to answer any questions that their friends may have!)
Maybe one of the reasons why the majority of teenagers abandon their Christian faith after they graduate from high school is that they've never been inspired, equipped and deployed to reach their own circle of friends for Jesus before they graduated from high school.
Mormon high school teenagers are trained at "Seminary" class every school day from 6am-7am so they can be better witnesses for the Mormon church. When they graduate they get a backpack, a nametag, a bike pump and a map to our neighborhoods. For two years these young missionaries knock on doors and share their version of the Gospel message. After two years of door to door work their faith has been steeled and sealed for the long term. Maybe that's why you don't meet a lot of ex-Mormons. They own their faith because it, like a stake, has been driven deeply into their souls by their outreach efforts.
We are the true church of Jesus Christ (of everyday saints!) And if teenagers are unleashed on an evangelistic mission in their own schools their faith will be steeled and sealed for the long haul. As they choose to lay it all on the line to share Jesus with their friends, not only will they be used by God to save the souls of their friends, but their own souls will be transformed in the process.
Daring teens to share and equipping them to do it doesn't just result in more lost souls being reached, it results in more Christian young people knowing, living and owning their faith for the rest of their lives.