John Chrysostom
Here the Apostle beautifully resolves the objection that had arisen. They could have said to him: if you promised to come to us but postponed your intention, and later change what you now say, as you did concerning your journey to us, then woe to us.
May the same thing not happen in the preaching itself! Therefore, so that they would not think this and be troubled, he says, "God is faithful, that our word to you was not yes and no."
In the preaching, he says, this did not happen, but only on the road and in journeys. In the preaching our words always remain firm and unchanging. For by "word" here he means the preaching.
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Then Paul presents an indisputable proof of this, referring the whole matter to God. His words mean the following: the promise to come was mine, that is, I promised this on my own.
But the preaching is not my affair and not human, but God's. What is from God is inaccessible to falsehood and deception. Therefore he said, "God is faithful," that is, true.
So do not suspect what comes from God and in which there is nothing human. Having mentioned the "word," the Apostle then adds an explanation of what word he means.