Albert Barnes on 2 Corinthians 1:18

But as God is true. Tindal renders this, in accordance more literally with the Greek, "God is faithful; for our preaching unto you was not yea and nay." The phrase seems to have the form of an oath, or to be a solemn appeal to God as a witness, and to be equivalent to the expression "the Lord liveth," or, "as the Lord liveth." The idea is, "God is faithful and true. He never deceives; never promises that which he does not perform. So true is it that I am not fickle and changing in my purposes." The idea of the faithfulness of God is the argument which Paul urges why he felt himself bound to be faithful also. That faithful God he regarded as a witness, and to that God he could appeal on the occasion.

Our word. Marg., preaching, (ολογος). This may refer either to his preaching, to his promises of visiting them, or his declarations to them in general on any subject. The particular subject under discussion was the promise which he had made to visit them. But he here seems to make his affirmation general, and to say universally of his promises, and his teaching, and of all his communications to them, whether orally or in writing, that they were not characterized by inconstancy and changeableness. It was not his character to be fickle, unsettled, and vacillating.

(1) "our word" "preaching"