The great Scottish professor, James Stuart Stewart, was voted by Preaching Magazine the best preacher of the 20th century, eclipsing even Billy Graham. In his book, Thine is the Kingdom, he discusses missionary motivation. Why do we go to the ends of the world? Why have we been doing this for 2000 years? Why does the church finance a global enterprise of unending expense and frequent privation and danger? Stewart suggests five reasons, but the first four of them are inadequate by themselves.
Commission. At the end of Matthew's Gospel, Jesus commanded us to go. This "Great Commission" is repeated in various ways in each of the Gospels and in the book of Acts. But this motivation, by itself, is inadequate. We don't engage in missions because of sheer obedience, and even if the Great Commission passages were lost to us, we'd still have a driving force within us to evangelize.
Compassion. Our love for a lost humanity motives missionary service. We're to rescue the perishing and care for the dying. But compassion will only take us so far.
Community. We are an obligation to the human family, whether they live in the same house, across the street, or around the world. Christianity emerges as the one true internationalism, says Stewart. Yet, this is not motivation enough.
Continuity. Someone brought the Gospel to us, and we have the obligation to pass it to a new generation. What if previous generations of missionaries had not passed the torch of truth to us? Yet commission, compassion, community, and continuity do not, by themselves, provide an adequate, sustained motive for our missionary enterprise. They represent major elements, but none of them touches the true profundity of the matter.
Christ. In the last resort, the one reason for missions is Christ. Belief in missions and belief in Christ stand and fall together. To accept Christ is to enlist under a missionary banner. A missionary outlook is a direct inevitable deduction from a saving knowledge of Jesus.