There is a very popular children's song that I believe is sung in churches all across North America. While the underlying purpose of the song can be encouraging, it is essentially not true. The song itself tries to encourage children to share the Gospel with anyone and everyone they know, even as young children, but it does so with words that are misleading. The popular song of choice? "Be A Missionary"
The simple truth is anyone can be a missionary but it does not mean that everyone is a missionary. Personally, I wrestled with this statement in college before finally understanding what that meant. After all, I was one of those kids that sung that song often and believed it to be true. One of my professors and I would speak at length regarding this topic and he would continually say, "we are saying the same thing, but we are using different words". And in this case, that word choice was very important. He helped me to see that not everyone is a missionary but everyone is a witness. The Bible teaches that everyone has a responsibility to go and share the Gospel with others - teaching them about the teachings of Jesus, and explaining the truth of salvation through His life, death, burial, and resurrection. It also clearly teaches that some of those witnesses can and will be missionaries.
Witness v. Missionary
A witness is someone that has special knowledge of something - in this case, truth. Therefore anyone that has accepted and believed that Jesus is the Way, the Truth, and the Life, and that no one can come unto the Father except through Him, is now charged to be a witness.
In Acts 1:8, Luke records Jesus charging the disciples to be witnesses; ". . .you will by my witnesses. . .". This just meant that those that had believed in all these things that Jesus had taught about were now responsible to go out and share them with others just as He charged them in Matthew 28:19-20.
On the other hand, missionaries are not just charged to share the Gospel but are called to specifically serve God in a new manner. This past Sunday we were talking about missionaries and one gentlemen at our church explained what I have been trying to figure out how to word in a simple but powerful way. I wish I could quote him but I do not recall his exact words. Essentially though he said, "A missionary is someone that gives up pursuing other callings and focuses their full time and attention to serve God by sharing the Gospel with others." As a missionary, one must be called as Saul and Barnabas were in Acts 13:2. They were called to be set apart by and for God. Then they were sent out by others and the Holy Spirit (vv. 3-4). Missionaries go wherever God calls them to go and proclaim the Gospel to everyone they can. Witnesses proclaim the Gospel, yes but they are not missionaries; not because they are not called but because they are called to something else!
Witness & Missionary
Looking at the differences of a witness and missionary requires also looking at the similarities.
Who: There is no restriction in terms of age, gender, etc. Everyone is to be a witness and anyone can be a missionary (again, the "restriction" here, being the calling).
Where: ANYWHERE. In Matthew 28:19-20 Jesus says, "Go". In Acts 1:8 Jesus says, "Jerusalem, Judea and Samaria, and to the end of the earth". Both of these are understood that it does not matter where you are, you are to be a bearer of truth - witnesses are to bear Christ as are missionaries - locally and globally.
What: Proclamation of the truth. Teach others the commands and how to obey them. Share with them the truth of the Gospel. Their sins were forgiven by Jesus' death and their eternal life was given by His resurrection. There is hope for them. Evangelize, disciple - teach them to do the same.
When: Right now. Anytime. From the point of salvation until death on earth. God has purpose for all me and that is to bear His image and tell of His truth. He desires all to be saved. Once one is saved, it is his duty to share that truth with others that they might have the same chance to know grace and to receive forgiveness and eternal life. God can also call people of any age, at any point in their lives, to devote themselves to the service of missions.
Why: Christ commanded it and He is the ultimate authority. His glory is to be shared on this earth by those that know of it and have experienced it to those that have not. The purpose is the same and that is what UNITES witnesses and missionaries.
How: Charge and calling. Missionaries and witnesses alike are charged and called to share the Gospel. Again, Acts 1:8 tells us this but the specific calling of ministry is where the line is then drawn.