1 Corinthians 14:22

Wherefore tongues are for a sign, not to them that believe, but to the unbelieving: but prophesying is for a sign, not to the unbelieving, but to them that believe. [The Old Testament generally is often called the Law by New Testament writers (John 10:34; John 12:34; Romans 3:20). Therefore the reference here is not to the Pentateuch, but to Isaiah 28:11-12. There the prophet tells how Israel murmured at the quality of the teaching which God gave them, and states that as a consequence God would soon teach them by the tongue of foreigners; i. e., the Assyrians would lead them away captive and they should be instructed by the hardships of captivity. When the captivity came, the necessity to understand and speak a strange tongue was a sign that God was teaching them, and yet a sign which they did not heed. From this incident Paul apparently draws several conclusions. 1. It was no especial mark of divine favor to have teachers who spoke an unknown tongue. 2. Tongues were for unbelievers and prophecy for believers. 3. Tongues were a sign that God was teaching, but the teaching itself was better than the sign. 4. Tongues, unless understood, had never been profitable; i. e., had not produced conversion. It must be remembered that Paul has in mind the abuse rather than the proper use of tongues. He illustrates his meaning by a hypothetical case.]