Christians are official representatives of Jesus Christ. It's in the Bible, II Corinthians 5:20, TLB. "We are Christ's ambassadors. God is using us to speak to you: we beg you, as though Christ Himself were here pleading with you, receive the love He offers you—be reconciled to God."
A Christian's life is just like an open letter. It's in the Bible, II Corinthians 3:2-3, TLB. "The only letter I need is you yourselves! By looking at the good change in your hearts, everyone can see that we have done a good work among you. They can see that you are a letter from Christ, written by us. It is not a letter written with pen and ink, but by the Spirit of the living God; not one carved on stone, but in human hearts."