A.T. Robertson Commentary 2 Corinthians 1:19

"For the Son of God, Jesus Christ, who was preached among you by us, [even] by me and Silvanus and Timothy, was not yea and nay, but in him is yea." — 2 Corinthians 1:19 (ASV)

Was not Yea and Nay (ουκ εγενετο να κα ου). "Did not become Yes and No."

But in him is yea (αλλα Να εν αυτω γεγονεν). Rather, "But in him Yes has become yes," has proved true. So Paul appeals to the life of Christ to sustain his own veracity.