2 Cor. 3:5-6 Not that we are sufficient in ourselves to claim anything as coming from us, but our sufficiency is from God, who has made us competent to be ministers of a new covenant, not of the letter but of the Spirit. For the letter kills, but the Spirit gives life. (ESV)
Paul goes on to explain further, fleshing out the qualities of a leader from 1:12 (simplicity, sincerity and dependence on grace). No one could claim to be sufficient for the calling to the mission of God (2:16). It is God that makes us competent, nothing of our own natural skills, talents and abilities could prepare us to be ministers of the new covenant in the Spirit (v. 6).
The new covenant is far superior to the old covenant of law (v. 7-11), it is permanent (v. 11) – of His kingdom there shall be no end. How could a guy like me be used in this which is even greater than the covenant of Moses? Because I am being transformed into the image of Christ (a glorious image); 
I am being conformed more and more to be like Him (v. 18). Where the Spirit of the Lord is, there is freedom (2 Cor. 3:17). How can we miss this? The new covenant is a covenant of the Spirit. The book of Acts makes that quite clear. It is the Spirit that gives life. Brad you have nothing to offer. Complete and utter dependence on the Holy Spirit is the most effective means to life transforming ministry to others.
Come Holy Spirit. It is you God who makes me competent! We are but ambassadors of You spreading the knowledge of You everywhere (2:14). If anything, may I be found sincere. Lord send me (2:17)!
In Jesus’ name – Amen,
Son
We are not even ambassadors. We are slaves, a word which our modern “democratic” society utterly rejects. We do not represent Christ, we follow the orders as handed to us by the Holy Spirit. And if we unconditionally and unquestionably do what the spirit tells us we are truly free because we are doing the will of God.
I was actually referring to 2 Cor. 5:20 where Paul says “we are ambassadors for Christ…” but (as you point out) he is also fond of the term doulas (slave or bondservant – a term he uses of himself many times in his letters) as well as the term diakonos (servant, minister or deacon).
Paul is my hero. I want to grow up to be just like him. Hope I live long enough.