First, because our own faith is at stake. Living faith is obedient faith (Matthew 28:19). A person who is called must walk in the light of that call.
Second, because all who have reached an age of moral responsibility have sinned and rejected the light (Romans 3:23; Isaiah 53:6).
Third, because people do not ordinarily walk in the light they have (Titus 2:11; Matthew 7:14). By personal contact a Christian missionary is able to encourage someone to do something they might not otherwise do. In other words, a person is more likely to be saved through Christian mission.
Fourth, all people are naturally corrupt and tend to pervert the truth they do have. Only by hearing the gospel can a person understand the deeper corruption that human nature bears. It is more likely they will overcome inherited depravity by hearing the Christian gospel.
Finally, apart from explicit knowledge of Jesus Christ, there is no Christian assurance. One must have knowledge of Christ to recognize the witness of His Spirit to our heart.
The gospel is the effective means that God has designed for saving the lost. It is the “power of God unto salvation.” But if a person can be saved without hearing the gospel, why should we send missionaries? Let’s consider some similar questions. If a person can be saved without my prayers, why should I pray for him? If a person has heard the gospel, why should we talk to him again? If a person can be saved without the church, why should we have churches? The obvious answer is, “Yes, he can be saved, but it is much less likely.” Even more so, missionaries find almost nobody who was saved before hearing the gospel. The fact that God will save a sincere person without the gospel shows His fairness, but it does not justify our neglect of the ordinary means He has designed. In general, those who do not hear the gospel will be lost. “Woe is me, if I preach not the gospel.”
The simple answer is that men, when they have seen the light, have rejected it. John 1:9 informs us that Jesus is “the light which lighteth every man.” But in the same chapter we read these dismaying words: “the light shineth in darkness; and the darkness comprehended it not” (v. 5); or, rather, did not lay hold of the light to possess it. Paul declares that creation made God known (Romans 1:19-20) and that, even when men “knew God, they glorified him not as God” (v. 21). John assesses the situation like this: “And this is the condemnation, that light is come into the world, and men loved darkness rather than light” (3:19). Man’s depraved heart prefers the darkness. Because of its deceitfulness and desperate wickedness (Jeremiah 17:9), the sinful heart turns from God.
Therefore, missionaries must take the light again! The God of second chances sends His ambassadors with the flaming torch of the gospel “to turn them from darkness to light” (Acts 26:18).
We send missionaries because the gospel message is best communicated relationally. The gospel truth is not a set of impersonal facts about God or religious belief boxes to be checked; it is embodied in the person of Jesus Christ. Missionaries are ambassadors for Christ, His message, and His ministry of reconciliation (II Cor. 5:20).
Missionaries are needed because without direct, relational communication of the gospel message, most will be lost. We don’t want to believe that people are being lost because of our inaction, but Romans 10:13-15 is clear: how shall they hear without a preacher? The sad truth is, they won’t. Sending and supporting missionaries is imperative. Hoping that God will do for us the work that He has called us to do isn’t a solution. God has given us the Great Commission, and to whom much is given, much is required. We send missionaries in response to God’s grace and in faith that God is at work in His world drawing people to His Great Heart.
An excellent question in light of some happenings in missions. For example, testimonies abound that in recent years many Muslims are coming to Christ after having dreams. I then ask, "Why doesn't the Lord provide more dreams as His tool of evangelism, thus not endangering many missionaries and dedicated local laymen living in Muslim locales?" Or, "Why doesn't He use angels who could avoid ‘harms-way’ situations, unlike we humans?" To answer this MissionSpeak question, I must respond to the "...why send missionaries?" question simply with, "Because God said so." I cannot box God in by saying He cannot work through dreams or angels or other means. He is God. No arguments with Him. However, the above-mentioned options (or any other avenues He chooses) are not the norm. His command is for us to "Go . . . and make disciples" (follow-up after conversion). This must be the heart-felt passion of the Church and not relegated to the level of a hobby!
1. Because Jesus told his followers to pray the the Lord would send forth laborers into his harvest (Mathew 9:38). Christ enjoined His followers to pray this way because it is His will to send laborers to lost people everywhere.
2. Because Jesus sends his followers into all the world (Matthew 28:19). Before ascending to heaven, Jesus commissioned twelve men who represented the Church to go and make disciples of all nations.
3. Because the early church obeyed the call of the Holy Spirit to send missionaries to other cultures (Acts 13:2). We would do well to follow its example.
4. Because the Spirit is still calling Christ-followers to be missionaries today. We are witnesses of and participants in His present-day sending.
5. Because the Bible is clear that sinful man rejects the light of God and therefore is guilty of sin and worthy of divine wrath (Romans 3:23). We send missionaries because all light-rejecters need to know Jesus Christ in order to be saved.