Paul now gives a last argument against the use of (uninterpreted) tongues in the worship service. This argument is based on Isaiah 28:11–12, which Paul refers to as the Law
(a term that is sometimes used to refer to the entire Old Testament; see, e.g., John 10:34; John 12:34; John 15:25).
The drunken priests and prophets of Israel were complaining that Isaiah spoke to them as if they were babies (Isaiah 28:9). This is how they viewed his message: “For it is precept upon precept, precept upon precept, line upon line, line upon line, here a little, there a little” (Isaiah 28:10). In Hebrew, the first part of Isaiah 28:10 sounds much like baby language: tsav lasav tsav lasav kav lakav kav lakav.
But Isaiah’s response was that, because they did not listen to his simple message, God would soon speak to them in the unintelligible language of the Assyrians (Isaiah 28:11–12). Paul quotes loosely from this prophecy, perhaps drawing a connection between the baby language described in Isaiah 28:1–29 and the tongue-speech of the Corinthians (see 1 Corinthians 14:20).
21 In the Law it is written, “By people of strange tongues and by the lips of foreigners will I speak to this people, and even then they will not listen to me, says the Lord.”