We are ambassadors representing our sovereign God to others. Therefore must be careful that the message we present is God's approved message. God is the only One who can reconcile man to Himself. Man cannot reconcile himself. Reconciliation was actually accomplished at the cross - but sinner must have response since they are at emnity with God. Verse 21 deals with what Luther called 'the great exchange'- Christ takes our sins that we may take His righteousness.