Note 26 at 1Co 14:22: Speaking in tongues is a sign to unbelievers, not believers. Usually a sign is a positive thing that helps people believe (see note 2 at Lu 2:12), but there are also negative signs that are intended to be stumbling blocks to those with hardened hearts (see note 10 at Mr 6:52 and note 3 at Mr 8:17).

Simeon prophesied over Jesus at His circumcision in the temple that Jesus would be a sign that would be spoken against (Lu 2:34). Isaiah also prophesied that the Messiah would be "a stone of stumbling," "a rock of offence," "a gin," and "a snare" (Isa 8:14). These prophecies certainly came true in many people's lives. Isaiah also said, in the scriptures that Paul had just quoted (Isa 28:11-12), that despite this miraculous manifestation, "they would not hear."

Therefore, speaking in tongues is a negative sign to unbelievers that will cause them to think we are crazy (1Co 14:23). That's the point Paul was making, and that is the reason he was telling them not to speak in tongues in church in a way that dominates the assembly (see note 13 at 1Co 14:28).

Someone might say, "But the disciples spoke in tongues on the Day of Pentecost, and 3,000 people were saved" (Ac 2:41). However, those tongues were the known languages of the people who were listening. Tongues with an interpretation equals prophecy (see note 12 at 1Co 14:5). Since those tongues were able to be interpreted by the hearers, that had the same positive effect that prophecy has (1Co 14:24-25).

So, Paul quoted Old Testament scripture to verify that tongues are a negative sign that will cause unbelievers to reject our witness. This was done to persuade the Corinthians not to speak in tongues in church without an interpretation.

Note 27 at 1Co 14:22: This verse is simply saying that tongues has a negative effect on unbelievers (see note 26 at this verse), while prophecy has a positive effect on those who will believe.