It does little good to encourage people to discipline themselves to evangelize if they do not know the gospel.
Try this experiment in your class or small group: Distribute notepaper. Ask participants how many times they think they've heard the gospel. Some, if they've professed faith in Christ for many years, may answer they've heard it hundreds or even thousands of times.
"Good!" you say. "Now, please write the gospel on that piece of paper."
Then watch people freeze, and stare at you as though you've asked them to list the names of every member of Congress.
"Didn't you just say that you've heard the gospel many times? And to become a Christian you had to hear the gospel and believe it to be saved, right? You can't be saved by a gospel you don't understand and believe. So please, simply write down in a paragraph or so the message people must hear to be made right with God and go to heaven."
Be prepared for an uncomfortable silence and for many blank sheets of paper - despite the presence of your best, most devoted members.
Repeating this exercise has shown that many committed church members are so unclear on the gospel that they cannot write it in their own words and in the simplest terms. How can we expect them to share the gospel in the world?
The gospel must be clear to believers before they can share it clearly with unbelievers.
This is not to say that a person must be able to articulate every nuance of the gospel to effectively witness, nor is it to imply that God cannot use an imperfect presentation of His gospel to save people. Moreover, there's no universally recognized, standard formulation of the gospel to which a person must subscribe. Nevertheless, I believe there's a widespread lack of clarity among professing Christians about the most basic aspects of Christianity's distinctive message. If believers have a tenuous grasp of the essential Christian message, how can they share it?
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.
So if these effects haven't occurred in the hearts of those who claim to have believed the gospel, at least one of two problems exists.
They have merely agreed that the gospel is true - thinking that agreement is saving faith - and have not in their souls actually relied on the gospel.
They simply do not understand it.
Evangelism: overflow and discipline
Despite the normalcy of gospel-changed people sharing the good news from the overflow of its effect in their lives, there's still a sense in which evangelism must be a discipline. It is easy for a "gospeler" to become so overwhelmed by responsibilities and burdens that he rarely finds himself in situation for meaningful conversation with non-Christians. Viewing evangelism as a discipline as well as a delight means that we sometimes choose to be with lost people - when we'd probably enjoy being with Christians instead - in hopes of talking about Jesus with them.
And let's not forget that the gospel is a message we communicate through words - words about the person and work of Jesus Christ. The discipline of evangelism is about intentionally speaking those words. While the consistency of our Christian example may affect the integrity of our witness, merely watching an example saves no one. Ultimately, it's not actions - important as they are - but the words of the "the gospel [that] is the power of God for salvation to everyone who believes" (Romans 1:16).
Jesus' Great Commission to us - to make disciples for Him of all nations (Matthew 28:19-20) - cannot be incidental or accidental. The intentionality of this supreme task implies discipline.
How might you intentionally create opportunities to speak of the life and work of Jesus?
Read more about Spiritual Disciplines and the Gospel.