When the man who had been possessed pleaded to remain, Jesus commanded him to go and announce all that the Lord had done for him. It seems rather odd for Jesus to turn away a potential follower, doesn't it? We would argue that it was with a thanksgiving spirit that the man wanted to remain with Jesus, something akin to the one leper who returned to thank Jesus (cf. Luke 17:11-19). As a matter of fact, in the account of the healing of the ten lepers, Jesus reprimands the other nine who did not return. Whatever the reasons for Jesus' response to the healed man in today’s Gospel account, the man did as Jesus commanded him: He went off and proclaimed throughout the ten cities (Decapolis) what Jesus had done for him. Maybe this was the intention of Jesus. Maybe Jesus had seen the potential in the man, and saw it better to make him a proclaimer of the good news. Had the man followed Jesus, maybe he would not have had the opportunity to preach the good news in the ten cities as he did (remember it was only after Jesus had been raised and ascended into heaven that the apostles actively engaged in apostolate). The man obeyed Jesus' command, the same command that Jesus would issue before his ascension into heaven: Go, make disciples of all nations (cf. Matthew 28:19a), and it was successful.
The man succeeded in the mission to the most part because he witnessed. Jesus commanded him to go and announce the good that God had done for him. The success of his proclamation can be attributed to the authenticity of the man's message. For he was not only a proclaimer of a message: he became the message itself. Not only did people listen to what he had to say, they also observed for themselves the transformation that he had undergone. They “saw” the message with their own eyes!
When Jesus sends us to go out on mission, this is what he expects of us: that we become the message that we intend to proclaim. We have to make the Good News of the Lord our Good News. What I proclaim should be about the good that the Lord has done for me. Only thus shall we be able to make disciples of all nations.