Matthew 28:19, 20 (NKJV)
In Matthew 28 we read about what is called "The Great Commission". When Jesus returned to heaven He left the work that He started to His followers. Jesus told His disciples that their job was to go to all the nations, make disciples, and teach. In Mark 16 it simply says to go into all the world and preach the Gospel, this is what the Great Commission is all about. However, if you look at what a lot of Christians are doing today it seems like they did not understand what Jesus was talking about. Most people who say they follow Jesus are not really doing what He asked them to do.
Jesus told His followers to go into the whole world, not "Stay where you are, focus on yourself, and do what you want to do". Many people would be happy if Jesus had said that, but it is not what He said. In Acts 1 Jesus said that the Holy Spirit would empower the disciples, and us, to be witnesses for Him, in Jerusalem, Judea, Samaria, and to the end of the earth. Again, Jesus is saying the same thing, go into the whole world. Yes, you can start with where you are but the Gospel message has to go out into the whole earth. This is what Jesus commanded and this is what we need to do.
Jesus did not give us "The Great Suggestion" and say if we can find the time it would be nice to do it. No, Jesus told us to go into every nation. Of course not everyone will relocate to another country, but we still need to do what we can to help fulfill the words of Jesus. Oswald J. Smith, who was a pastor and a missions advocate, once said, "You must go or send a substitute". All of us are called to reach the world but not all of us will personally go to the whole world. But, we can pray, we can give financially, and we can support those who do go. Individually we cannot fulfill this, but together, with each one doing their part we can fulfill The Great Commission!