2 Corinthians 1:18

But as God is true
It seems that the false apostles had insinuated, that as the apostle had not kept his word in coming to them as he had promised, that he was not to be depended upon in his ministry; that he might as well contradict himself, and deceive others in the one, as well as in the other: wherefore he appeals to God in a very solemn manner, calls him to witness to the truth of his doctrine; for these words may be considered as the form of an oath; or he argues from, the truth and faithfulness of God, to the certainty and invariableness of the word preached, who is so true and faithful as that he will never suffer his word to be yea and nay: for when the apostle says, that

our word towards you was not yea and nay,
he does not mean his word of promise to come to Corinth; but the word of his preaching, the doctrine of the Gospel, which was not uncertain, changeable, sometimes one thing, and sometimes another, and contradictory to itself. And by this the apostle would intimate, that since he was faithful and upright, uniform, consistent, and all of a piece in preaching the Gospel to them; so they ought to believe, that he was sincere in his resolutions and promises to come and see them, though as yet he had been hindered, and had not been able to perform them.