That word is evangelism. But as embarrassing as that word and its practice might be, the Presbyterian Church still sees evangelism as the primary, the first, of the Great Ends of the Church what we are supposed to be about and doing each and every day. In its simplest terms, evangelism is simply telling somebody else the incredibly joyful news about the love of God and his Son, Jesus Christ. Its just telling your friend or neighbor about Jesus. John Fisher has a wonderful image for evangelism.
Evangelism is just one beggar telling another beggar where to find food.
The main reason is, because Jesus himself told us to go and do it. We have, directly from our Lord and Savior, a Mandate to Evangelism. Here the resurrected Jesus tells his disciples what they are to be doing during the interval, while they await Christ's return to earth. And surely I am with you always, to the very end of the age. So the reason we're so concerned about evangelism, the reason its such a big deal, is because Jesus told us to go and do it.