Charles Ellicott Commentary 2 Corinthians 1:18

"But as God is faithful, our word toward you is not yea and nay." — 2 Corinthians 1:18 (ASV)

As God is true.—Literally, as God is faithful. The words were one of St. Paul’s usual formulas of assertion. (Compare to 1 Corinthians 1:9; 1 Corinthians 10:13; 2 Thessalonians 3:3.) In other instances it is followed commonly by a statement regarding some act or attribute of God. Here it is more of the nature of an oath: “As God is faithful in all His words, so my speech” (the vague term is used to include preaching, writing, personal interaction) “is true and faithful also.” There had been no “Yes” and “No” in the same breath; no saying one thing when he meant another.