This passage occurs on Ash Wednesday (Year A) and also Year C, Lent 4.
Summary: There is so much going on in this passage! This is my first time preaching on these words and so my comments below are my first observations!
Things I am observing
1) The power of verse 21 should not be lost in translation. This is a radical transformation of Christ to become sin. This verse is crucial (get it, crucial) in understanding Luther's thought. Paul makes a claim about how Jesus fully embraces our humanity here to the point of becoming sin. (Not simply a sinner, but αμαρτια, sin itself). It would take centuries for people to realize just how amazing Paul's claim here is.
2) For Paul, the "therefore" of ministry is never far away from the "what God has done for you in Christ." 6:1 is a really harsh call to action -- "Don't let the grace of God become empty (κενος)." Paul knows this word, for in Philippians he claims that Christ first emptied himself for us.
So, it is worth asking ourselves then -- what does it mean not to take the grace of God in vain?
Paul calls forth a life of servanthood -- διακονια. While the modern conception of "deacon" is sometimes solely focused on acts of mercy and hands on service, Paul here links the idea of διακονια with two things
a) the ministry of sharing the good news of Jesus
b) suffering.
3) A big debate within theology for the last 500 years at least as been whether we are to start with the law and conclude with the Gospel or start with the Gospel and conclude with the law. However, what I want to point out is that while Paul does here move from Gospel (he became sin for you) to Law (don't let the grace of God be in vain), he does not end up with a legal schema. The life of the Christian is not simply obedience to the commandments, but discovery of God's resurrection power amid life's suffering. The fruit of the Spirit is not that we all fit the mold, but that we discover the cruciform life is actual life, life that cannot be contained or controlled.
4) 5:20 presents a translation challenge. Paul writes, "We are ambassadors for Christ, "ως του θεου παρακαλουντος δι' ημων"
What is tricky is the word "ως" which can mean "like" or "how" or "as." Some people want to make this "as" in a "as if" kind of way:
as though God were making his plea through us. (NET)
Other wants to make this "as" to say "as by means of": God making his appeal through us (NRSV, ESV)
I think that both of these translations, while faithful, do not fully capture what I see as Paul's point -- God is the one doing this work. Furthermore, I think the word "appeal" is a really watered down translation of παρακαλεω, which also means to comfort, encourage and console. Appeal might seem like we have to argue Christ's case rather than be ministers for Christ's sake. So I propose:
"We are ambassadors for Christ as God comforts through us"