"Can a person genuinely understand and believe the gospel and not want to share it with others - not seek ways to spread it? The gospel is self-perpetuating. The Holy Spirit works through the good news to create spiritual life in a dead soul. He then works through this new life to spread the message of Jesus again. New believers love the gospel in such a way that he or she wants to share its message.
The effect of the gospel on the believer's heart is to create a 'gospeler' - a person who wants to evangelize, who wants to tell others about the person and work of Jesus Christ. Notice that I said the person wants to evangelize. For various reasons he may often fail to do so, but the desire is present. And the desire isn't based on merely wanting to live up to expectations. Rather, there is a genuine longing to see people become followers of Jesus."
Taken from http://www.lifeway.com/ArticleView?storeId=10054&catalogId=10001&langId=-1&article=spiritual-disciplines-gospel-evangelism-witnessing-missions-salvation-sharing-faith .