Charles Ellicott Commentary 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, saith the Lord." — 1 Corinthians 14:21 (ASV)

In the law it is written.—The preceding teaching is illustrated and reinforced by an appeal to Jewish history. The Old Testament as a whole was often called “the Law” (John 12:34; John 15:25).

The words are not exactly a quotation, but rather an illustration taken from Isaiah 28:9-12. That passage refers to Israel's refusal to listen to Jehovah when He spoke to them clearly and simply. His judgment on them took the form of declaring that He would make a foreign people—the Assyrians—His mouthpiece to them in the future, in a language they did not know.

It is as if the Apostle said: Remember there was a time in Jewish history when an unintelligible language was a sign sent by God, and it proved ineffective for Israel's conversion. The gift that you now exalt so highly is, by itself, equally useless for that same purpose.