A.T. Robertson Commentary 2 Corinthians 3:6

"who also made us sufficient as ministers of a new covenant; not of the letter, but of the spirit: for the letter killeth, but the spirit giveth life." — 2 Corinthians 3:6 (ASV)

Who also made us sufficient for such confidence (ος κα ικανωσεν ημας). Late causative verb from ικανος (verse 5) first aorist active indicative, "who (God) rendered us fit." In N.T. only here and Col 1:12.

As ministers of a new covenant (διακονους καινης διαθηκης). Predicate accusative with ικανωσεν. For διαθηκη see on Mt 26:28 and for διακονος on Mt 20:26 and for καινης (fresh and effective) on Lu 5:38. Only God can make us that.