What does Mark 5:20 mean?

The man whom Jesus saved from the legion of demons had lived in a graveyard in the hills above the eastern shore of the Sea of Galilee. The larger area, east and south, was known as "Decapolis" or "ten cities." To us, the area is the Golan Heights, northwest Jordan, and southwest Syria.

This is the first time in Mark that Jesus has told someone to spread news of what He has done, and it's in a Gentile area. Ironically, the Gentiles are able to see Jesus as He is, without the weight of the prophecies of the Jewish Messiah. The Jews of Jesus' time anxiously await the coming of their savior, as they should, but they don't understand what the Messiah will save them from. Much of the Messianic prophecies in the Old Testament focus on God's restoration of Israel as a strong, respected, independent nation. Nations will bring them riches, and a king from the line of David will sit on the throne.

When the Jews begin to understand Jesus is the Messiah, they expect Him to make all these prophecies come true in their lifetime. They don't understand that God's promises won't completely come to fruition until the Millennial Kingdom, far in the future. The Gentile view of Jesus is simpler. Instead of facing the Messianic expectation as a roadblock for understanding, the gospel can work in Gentile hearts by introducing a Man who heals injuries and illnesses and saves people from demons, and explaining that He also heals our relationship with God and saves us from sin.

This is the message that the man from the tombs begins to spread around Decapolis. Despite the people's initial misgivings (Mark 5:15–17), they will soon welcome Jesus (Mark 7:31–37). And in the time of the apostles, Christianity will spread.