What role do believers have on earth according to Paul?
Paul says believers are left on earth as ambassadors of their homeland, heaven. They serve as representatives of that heavenly government while living here, just as an ambassador represents his nation abroad. Because they belong to the Kingdom of Christ, they are now citizens of heaven.
Les explains that Paul gives believers the same kind of role as an ambassador living in a foreign country. Just as an American living abroad is a representative of the government in Washington, and foreign service people are to be visible representatives of their government, believers are left here as ambassadors of their homeland, which is in heaven. Les also refers to the illustration of The Ugly American, where ambassadors and foreign service people should be recognizable representatives, though some were not good representatives. Paul then ties this to Colossians 1:12, where the Father has made us meet to be partakers of the inheritance of the saints in light, and has delivered us from the power of darkness and translated us into the kingdom of His dear Son. That kingdom is in heaven, so believers are now citizens of heaven because they are members of that Kingdom of Christ.