We are therefore Christ's ambassadors, as though God were making his appeal through us. We implore you on Christ's behalf: Be reconciled to God. (2 Cor 5:20)We usually think about evangelism in terms of "witnessing", and with good reason: Christ appointed his apostles as his witnesses, and we carry on their work. Witnesses are people who can personally guarantee the truth of what they report: they discuss what they see or hear, what they know. When we hear of witnessing, we often think in terms of testimony and court arguments. This has often been the case for Christians, as our beliefs are at times persecuted by law.
Paul gives us a second way to look at the same job of being Christ's messengers: we are ambassadors. An ambassador is a foreigner. We are called to picture ourselves as foreigners even in our own culture. That's true enough, as we have different views on everything from sexual integrity to self-control to how to treat our enemies. Ambassadors -- and foreigners in general -- stand out for their differences. They don't fit in. They eat differently, speak differently, dress differently, have different custom and different habits. And yet they are not embarrassed by any of that: they are true to the country in which they are citizens.
The ambassador's job is to represent the ruler who sent them. They speak on his behalf, representing his interests and not their own. Usually ambassadors are sent to make peace or keep peace. It can lead to different conversations than the "witnessing" model of evangelism. Both are the good news of Christ.
Paul focuses us on the message with which we have been sent:
Therefore, if anyone is in Christ, the new creation has come: The old has gone, the new is here! All this is from God, who reconciled us to himself through Christ and gave us the ministry of reconciliation: that God was reconciling the world to himself in Christ, not counting people’s sins against them. And he has committed to us the message of reconciliation. We are therefore Christ’s ambassadors, as though God were making his appeal through us. We implore you on Christ’s behalf: Be reconciled to God. God made him who had no sin to be sin for us, so that in him we might become the righteousness of God. (2 Corinthians 5:17-21)