1 Corinthians 14:21
“In the law it is written, By men of strange tongues and by the lips of strangers will I speak unto this people; and not even thus will they hear me, says the Lord.”
Yet surely it is no where written in the Law, but as I said before, he calls always the whole of the Old Testament, the Law: both the prophets and the historical books. And he brings forward his testimony from Esaias the prophet, again covertly detracting from the glory of the gift, for their profit; nevertheless, even thus he states it with praise. For the expression, “not even thus,” has force to point out that the miracle was enough to astonish them; and if they did not believe, the fault was theirs. And wherefore did God work it, if they were not to believe? That He might in every case appear to do His part.
2. Having shown then even from the prophecy, that the sign in question is not of great use, he adds,
Source: Homilies on First Corinthians (New Advent)