Graphic derived from 2 Corinthians 5: 20 - 21: "Now then, we are ambassadors for Christ, as though God were pleading through us: we implore you on Christ’s behalf, be reconciled to God. For He made Him who knew no sin to be sin for us, that we might become the righteousness of God in Him."
 
An ambassador is someone sent on behalf of someone (e.g. their nation) as a representative elsewhere. We, as Christians are citizens of heaven and God has put us on earth for the time being to represent Him to the nations. To reveal His character so others will come to know Him.