We know no man according to the flesh... That is, we consider not any man with regard to his nation, family, kindred, or other natural qualities or advantages; but only with relation to Christ, and according to the order of divine charity, in God, and for God. The apostle adds, that even with respect to Christ himself, he now no longer considers him according to the flesh, by taking a satisfaction in his being his countryman; his affection being now purified from all such earthly considerations.
5:17. If then any be in Christ a new creature, the old things are passed away. Behold all things are made new.
5:18. But all things are of God, who hath reconciled us to himself by Christ and hath given to us the ministry of reconciliation.
5:19. For God indeed was in Christ, reconciling the world to himself, not imputing to them their sins. And he hath placed in us the word of reconciliation.
5:20. For Christ therefore we are ambassadors, God as it were exhorting by us, for Christ, we beseech you, be reconciled to God.
5:21. Him, who knew no sin, he hath made sin for us: that we might be made the justice of God in him.
Sin for us... That is, to be a sin offering, a victim for sin.
2 Corinthians Chapter 6
He exhorts them to a correspondence with God's grace and not to associate with unbelievers.
6:1. And we helping do exhort you that you receive not the grace of God in vain.