2 Corinthians 5:20 "We are therefore Christ's ambassadors, as though God were making his appeal through us."
From IVP Bible Background Commentary: An Ambassador was usually a representative of one state to another, usually applied in this period to the emperor's legates in the East. This image fits "apostles" as appointed messengers, just as Old Testament prophets has been. In the context of a plea for reconciliation, Paul as an ambassador urges the Corinthians to make peace with God the King; emperors normally took action against unrepentant client states that offended them, and no one took such warnings lightly.
Information on the modern system of diplomacy: the wikipage on Diplomatic ranks.