Because Christ is Lord of all, there is no one throughout the world who is not obligated to render to Jesus the glory that belongs to him as the crucified, risen, and ascended Savior and Judge. As Christians share the message of salvation, both near and far, the Lord uses their proclamation of the gospel to save sinners. In the wake of these conversions is worship of the triune God where it did not take place before.
Tertullian, writing around A.D. 200, described the situation in his own day: "We are but of yesterday, and we have filled every place among you—cities, islands, fortresses, towns, market-places, the very camp, tribes, companies, palace, senate, forum,—we have left nothing to you but the temples of your gods." The Christian church continues to do that. Like the small mustard seed, the kingdom of God grows as the gospel is faithfully shared with others wherever Christians are found.
Why Evangelize? And How?
In these attempts to communicate with others, however, the response is often hostile and indifferent. So why do it? Why go to all the trouble just to be mocked and rejected the vast majority of the time? Why not spend more of our money and efforts trying to disciple those who are already Christians? Or if we want to have at least something to do with evangelism, why not wait for interested people to come to the church, rather than experience the persecution that awaits us in going outside it? We could perhaps come up with a number of reasons why we should care about sharing the gospel with others, but we will mention just two. The first is so simple that it might not be the first reason we give. We are to evangelize because Jesus commands it.
When R. C. Sproul was a seminary student, he took a class from Dr. John Gerstner, who was a professor of church history. There were about eighteen students in the room, sitting in a semicircle. They were discussing predestination, and Gerstner asked a question, beginning with the student at the opposite end of the semicircle as Sproul: "Now, sir, if predestination is true, why should we be involved in evangelism?" One student after the other did not know the answer to this question.
Meanwhile, as Sproul is watching all these failures, he is starting to feel the pressure as Gerstner is getting closer to Sproul's end of the room. "Well, Mr. Sproul," asked Gerstner, "if predestination is true, why should we be involved in evangelism?" Sproul felt himself sliding down in the chair and with all kinds of apologies and qualifiers threw out his answer: "Well, Dr. Gerstner, I know this isn't what you're looking for, and I know that you must be seeking for some profound, intellectual response which I am not prepared to give. But just in passing, one small point that I think we ought to notice here is that God does command us to be involved in evangelism."
Gerstner laughed and replied, "Yes, Mr. Sproul. God does command us to be involved in evangelism. And of course, Mr. Sproul, what could be more insignificant than the fact that the Lord of glory, the Savior of your soul, the Lord God omnipotent, has commanded you to be involved in evangelism?" Sproul concluded, "I got the point in a hurry!"
The second reason we are to evangelize is the wrath of God. People worry about many things. They think about their family's health and safety, and whether their children will turn out alright. They worry about their job and whether it is secure. They ponder their financial situation and whether or not they can afford whatever the next major expense will be. They look ahead to retirement and question whether they have enough money put away to make it. They daily observe their own appearance, noticing with alarm that they are older than they used to be, and there is nothing they can do to stop it.
But the one thing—the most important thing—that very few people pay any thought to is who God is, what he expects of them, where they learn about him, and whether or not he just might be angry with them. People go along through life trying to dodge one problem after another, not realizing that not only is God angry with them as sinners, but also that this angry God is coming in judgment to deal with them. We are to evangelize to tell them of their only way of escape and how to prepare for this God's arrival.
At the end of Matthew 28, verses 18-20, not only does Jesus tell us to evangelize, but he also tells us how. We are to make disciples. Those who believe the gospel and repent of their sins are to follow Jesus as Lord. This command is marked by two items. The first is baptism, which here is seen as a public act of identification with Christ before other Christians and the world. The second is that our evangelism is to include teaching others all that Jesus has commanded, what we might call discipleship.
This brings out an important point about witnessing. According to Matthew 28, evangelism is not strictly sharing the gospel in the hope of simply making converts and then moving on to other people. One cannot isolate evangelism as merely telling others about the message of salvation without also teaching them all that Jesus commands. Thus, evangelism is not really evangelism if it is not involving discipleship so that new Christians can grow in their knowledge of God and what the Christian life entails.
From this passage, not only does Jesus command us to evangelize and tell us how to do it, but he also gives two promises to provide strength along the way. The first promise is that all authority in heaven and on earth has been given to him. He is the resurrected Lord, and there is no place where his sovereign authority does not reach. He is the Savior of the world and the Judge of all the earth. One can respond to this in a number of ways. One can deny it and conclude that it is all just a lot of religious nonsense, useful only for those who are too weak and insecure to get through a hard life on their own. One can ignore his claim of universal dominion, believing that while not necessarily irrelevant, still there are more important priorities and interests. Or one can bow before him, acknowledging his claim on them and pursuing the salvation that he graciously offers as one relentlessly seeks priceless treasure buried in a field.
The second promise is Jesus' continuing presence, even to the end of the age. If you are a Christian, no matter where you are, Jesus promises to be with you. He will never leave you because this promise of his abiding presence has been enacted by his own blood shed for your sins, with his Holy Spirit given to you as a seal that will never break away. This gives us confidence and joy in our witness, regardless of the rejection and persecution we might face. And as the Spirit of Christ empowers us to share the gospel with others, we must do it with the compassion that Jesus himself showed in his lament over Jerusalem for her rejection and unbelief (Matt. 23:37).
When D. L. Moody was evangelizing in various cities throughout Great Britain, some clergymen were jealous of his powerful preaching and wanted to know his secret. They said to him, "Mr. Moody, we would like to have a word with you. You come here to London, you have a sixth grade education, you speak horrible English, your sermons are simple, and yet thousands of people are converted. We want to know, how do you do it?"
Moody proceeded to invite them in and asked them to look out the window and tell him what they saw. They mentioned some things going on outside, such as some children playing in a park, and some couples walking. Then they asked Moody what he saw.
According to the account, as Moody looked outside, tears began to roll down his cheeks and onto his gray beard. One of the ministers, very curious to know what caught Moody's eye that would so affect him, asked, "Mr. Moody, what are you looking at? What do you see?"
Moody replied, "When I look out the window, I see countless thousands of souls that will one day spend eternity in hell if they do not find the Savior."
Christians ought never to be smug, but always to speak and act as the beggars that they are, fully convinced that the only difference between them and the rest of the perishing world is that by God's sovereign mercy they have come to learn where their bread is found, and they want others to know it too. Christians have received the greatest message there is, the best news that anyone can ever hear. How will you share it with others before it is too late?
 Tertullian, Apology xxxvii, Ante-Nicene Fathers, ed. Alexander Roberts, and James Donaldson, 10 vols. (Peabody, Massachusetts: Hendrickson, 1995), 3: 45. Original publication, Christian Literature, 1885.
 R.C. Sproul, "Prayer and God's Sovereignty," in Our Sovereign God, ed. James M. Boice (Grand Rapids, MI: Baker, 1977), 127-128.
 Recounted in Lane T. Dennis, "What Do You See?" Share the Good News (September/October 1999), 3. Quoted in Philip G. Ryken, The Message of Salvation (Downers Grove, IL: InterVarsity, 2001), 293-294.