Jamieson, Fausset and Brown on 2 Corinthians 3:4

4. And--Greek, "But." "Such confidence, however (namely, of our 'sufficiency,' 2Co 3:5, 6; 2Co 2:16--to which he reverts after the parenthesis--as ministers of the New Testament, 'not hinting,' 2Co 4:1), we have through Christ (not through ourselves, compare 2Co 3:18) toward God" (that is, in our relation to God and His work, the ministry committed by Him to us, for which we must render an account to Him). Confidence toward God is solid and real, as looking to Him for the strength needed now, and also for the reward of grace to be given hereafter. Compare Ac 24:15, "hope toward God." Human confidence is unreal in that it looks to man for its help and its reward.