2 Corinthians 5:16-21 is a crucial passage not only in the Corinthian correspondence, but in the Pauline corpus as a whole. This is primarily because it is laden with Pauline themes, such as new creation, in Christ, reconciliation and the righteousness of God. Paul speaks about not regarding anyone from a human point of view, this is the case because Paul initially knew Christ from this perspective. This does not mean that Paul had no interest in the historical Jesus as some scholars have suggested, rather, “What Paul is saying is that previously he had a completely inadequate knowledge of Christ—one based on a human point of view—but not now, his understanding of Christ is no longer limited in that way.”
Knowing Christ kata sarka, Paul would have shared with other 1st Century Jews who expected Jesus to be a political and military messiah. The result was the failure to see Christ as the Messiah on God’s mission, reason being that he was using human eyes rather than God’s eyes. As expected this led him to the wrong conclusion about Christ. Paul changed his perspective, on the road to Damascus, after his encounter with the risen Christ. Paul no longer sees as before, as J. Paul Sampley states, “the old ways of looking, perceiving, understanding, and, more profoundly, evaluating, have to be let go and replaced with a new way of seeing and understanding.” Knowing kata sarka, is not unrelated to living kata sarka just as knowing kata pneuma is related to living kata pneuma.
In verse 17, Paul states that “If anyone is in Christ, there is a new creation”. Murray J. Harris has summarized the possible readings of this phrase:
“(1) γίνεται … ἔστιν, “So, if anyone comes to be in Christ, there is a new creation” (Martin 135; similarly Moffatt);
(2) ἐστιν … ἔστω, “Therefore if any man be in Christ, let him be a new creature” (KJV)
(3) ἐστιν … ἔστιν, “So, if anyone is in Christ, there is a new creation” (Furnish 306);
(4) ἐστιν … ἐστιν, “Therefore, if any one is in Christ, he is a new creation” (RSV, NIV)” 
Space does not allow us to discuss the merits and demerits of each. However option three, following Furnish and the NRSV, seems to be the best rendering of the grammatical construction in Greek. To be “in Christ” has to do with both being “in personal union with the risen Christ” and “in the body of Christ”, for Harris, “Neither the individual nor the corporate dimension of the phrase should be ignored.” ἐν Χριστῷ (In Christ) is what has made the difference, and the result is καινὴ κτίσις (new creation). For Paul this is a “new act of creation” which God has undertaken through Christ, meaning that Paul does not imply an individual subjective view. However, it is not to say that the individual does not experience the newness of life, for Paul new creation is the basis for the newness of life (Rom 6:4). In Paul's view, new creation nullifies the old order, it makes everything new. Even though, the old order, seems to linger, this reality will not be for long. The day is already dawning and the night is already over (Romans 13:12-14). In the meantime those in Christ, live in what scholars have dubbed the “already and not yet”.
Paul proceeds to reveal the source of this new creation, it is none other than God himself, who is Christ has reconciled all of us to himself. Reconciliation in this case is an accomplished fact, as seen in the aorist participle καταλλάσσω. This does not imply that there no need for reconciliation to take place on the part of human beings, this tells us that the objective aspect is done, it is now left for humanity to do the subjective aspect. This is the reason Paul speaks about the ministry of reconciliation, which is, “primarily the proclamation of what God has done.” The object of God’s reconciling work is “us” (possibly Paul, his co-workers”, and “the world” possibly the rest of humanity since verse 19 says “not counting their trespasses against them” the world cannot possibly have sinned against God, we can safely conclude that humanity is implied here. Unlike in Romans where justification and reconciliation are linked (Rom 5:9-10), “Justification has its social setting in the law or in the courts, and so it does not have any pertinence here in 2 Corinthians; reconciliation has its social setting in a familial or friendship environment where there has been a restoration of a broken relationship, precisely Paul’s recent and somewhat continuing problem with the Corinthians.” Understood from this perspective, this passage fits in with Paul’s agenda in writing the letter.
Related to having the ministry of reconciliation, is Paul’s ambassadorial role mentioned in v.20. Paul does not represent a human king, but the God of the universe, who has endorsed and given him authority to speak for him. As an ambassador of God, Paul would have been commissioned to represent God and exercise authority on his behalf. As such he was to be “treated with respect and dignity, accorded appropriate hospitality, and guaranteed a safe exit.” This Corinthians were yet to come to terms with this understanding, this was because of the supper apostles. Paul worked both “on behalf of Christ” and “in the place of Christ”, hence there is no need to choose between representation and substitution since in ὑπέρ both aspects are present. On that basis Paul makes an appeal on behalf of God, “be reconciled to God” the word used here is in the imperative mood. As to who Paul has in mind, three options have been advanced, that is unbelievers, Corinthian believers and evangelistic audience. There is no need to choose one or the other, since the Corinthian church has a wide variety of people at different points in their spiritual journey, some of whom are causing trouble for Paul, these need to be reconciled to God. Any person who has not been reconciled, ought to heed Paul appeal, “be reconciled to God”.
In verse 21, Paul concludes by asserting that Christ, was made to be sin, so that we may become the righteousness of God. Christ, who knew the reality of sin, did not personally entangle himself with sin, and the focus of Paul assertion is Christ’s earthly ministry rather than his pre-incarnate existence. Paul stands with the rest of Christian tradition, in asserting the sinlessness of Christ. It is what Christ did that becomes the basis for believers becoming righteousness of God.
 Colin G Kruse, The Second Epistle of Paul to the Corinthians: An Introduction and Commentary (Leicester, England; Grand Rapids, Mich.: Inter-Varsity Press ; Eerdmans, 1987),125.
 Margaret E Thrall, The First and Second Letters of Paul to the Corinthians: Commentary (Cambridge: Univ. Press, 1965),149.
 Kruse, The Second Epistle of Paul to the Corinthians,125.
 J. Paul Sampley, The second letter to the Corinthians New Interpreters Bible (Eds. Leander E Keck and J. Paul Sampley; Nashville: Abingdon Press, 2007), 93.
 Murray J. Harris, The Second Epistle to the Corinthians: A Commentary on the Greek Text, New International Greek Testament Commentary (W.B. Eerdmans Pub. Co.; Paternoster Press, 2005), 430-31.
 Harris, The Second Epistle to the Corinthians, 431.
 Ralph P Martin, 2 Corinthians Word Biblical Commentary(Waco, Tex.: Word Books, 1986),152.
 Robert C Tannehill says For Tannehil, new creation and being in Christ is not subjective, for “If paul were only able to assert that ‘for me’ or ‘in my view’ the old world has passed away, he would not be able to argue that others may no longer judge him according to the flesh, for they would be as entitled to their viewpoint as he is to his. Paul’s own argument in these verses depends upon the reality of the presence of the new aeon.” in Dying and Rising with Christ. A Study in Pauline Theology (Berlin: Töpelmann, 1967),68-9.
 Kruse, The Second Epistle of Paul to the Corinthians,127.
 Sampley, The second letter to the Corinthians, 95.
 III Witherington Ben, Conflict and Community in Corinth: A Socio-Rhetorical Commentary on 1 and 2 Corinthians (Wm. B. Eerdmans Publishing Co., 1995), 396.
 Harris, The Second Epistle to the Corinthians, 445.
 Harris, The Second Epistle to the Corinthians,446
 Harris, The Second Epistle to the Corinthians, 447-48.
 Harris, The Second Epistle to the Corinthians, 450.