When the Law was given at Mt. Sinai 3,000 people were killed
Q. What does it mean when 1Cor 12.22 says that tongues is for the unbeliever?
A. To understand what this means, we have to go up to v 21 and see that Paul is quoting Isa 28.11.
In the context of the verse, Isaiah says that the Lord had spoken plainly to the people through the prophets, in their own language, so that even a child could understand, but they didn’t listen.
So, He threatens them with another method.
He would speak to them through the Babylonians and the Medes when they take the city.
Later it would be the Greeks and Romans as history plays out up to the first century.
Now, there was no reason for the Corinthians to be so desirous of "tongues" because they have been used by God in "punishment" form, not in a way of blessing.
Let me give an example of this.
In Acts 2. 1-4 we have the Holy Spirit given to believers who were gathered in the Temple on the Feast of Shavuot, or "Pentecost."
This was the day that God gave the Law at Mt Sinai (Exo 19.1-17).
In v 5-11 the unbelievers in Yeshua were amazed because these disciples were speaking the known languages from wherever they were from.
In v 12-15 they wondered what all of this was about and starting in v 16-36 they were told by Peter that it was a sign from God that the Spirit was being poured as another proof that Yeshua was the Messiah who was delivered up to them according to a predetermined plan and they murdered him, but God raised him from the dead.
The tongues prompted their questions and Peter was able to preach to them in a language they all could understand.
In v 37-41 the people are shocked and they ask what they should do.
You see, the tongues got their attention.
Peter tells them to repent of their sins and be immersed (baptized) and receive the Holy Spirit.
So, being in the Temple, they go over to the mikva’ot (immersion baths) located at the southern end of the Temple mount and three thousand were saved/baptized.
Now, what is interesting is that when the Law was given at Mt Sinai, three thousand people were killed (Exo 32.28).
In Acts 2, they are gathered in the Temple on the day they celebrated the giving of that Law and 3,000 people are saved.
You see, the Law in and of itself cannot save anyone, it kills, but the Spirit of God gives life.
In short, the people Isaiah was dealing with rejected the plain truth given by the prophets.
Because they rejected it, God allowed strange tongues through the Babylonians to enter the city and take the people captive.
They would believe the prophets now because of the strange tongues they were hearing in the streets.
In Acts 2, the people were rejecting the plan truth given by the prophets again, and ultimately the Messiah Himself.
As a result, they hear tong-ues in the city again as a sign of judgment.
The people need to repent and accept Yeshua as the Messiah, corporately as a nation.
But, we know they don’t and in 70 AD God sends a foreign nation with foreign tongues against them (Rome and all her auxiliaries), like the Babylo-nians, and destroys the city and the Temple.
Not by coincidence.
The Babylonians and the Romans take the city and destroy the Temple on the same day of the Jewish calendar, the 9th of Av.
So, that is why Paul says that tongues are a sign for the unbeliever.
There is much more to this but I hope this helps.