1 Cor 14.22 says that tongues is a sign for the unbeliever, but what does that mean exactly. Some teach that when an unbeliever goes into a congregation and they hear someone speaking in tongues, they will believe because of what they see. That could certainly happen, but that is not what this verse is referring to.
In v 21 Paul quotes Isa 28.11 where it says that he will speak to this people (Israel) through stammering lips and a foreign tongue. The context in Isaiah is that the Lord had spoken plainly to the people through Isaiah and the other prophets very clearly, and in their own language, so that even a child could understand (28.9). But, they didn’t listen. In Isa 28.10 he says in Hebrew, “tzav latzav, tzav latzav, qav kaqav, qav laqav, zeer sham, zeer sham.” This is translated in English “order on order, order on order, line on line, line on line, a little hear, a little there.” Many have taught that this is how God teaches us, but thus is not the meaning. What the Lord is communicating through Isaiah is that they will hear meaningless gibborish and senseless babbling, a syllable they will recognize here, a syllable they will recognize there. It will be as if they are hearing a foreign tongue. This is a chiding phrase and not talking about God’s wisdom, but a travesty of God’s wisdom that is going to bring God’s judgment. They reduced the Torah to a shambled pile of little precepts, like a child who speaks gibberish.
So he tells them that there was going to be another method that the Lord will use to get their attention. He was going to speak through the Assyrians and the Babylonians (and others) who are coming to take the land because they were not listening. Later, it would be the Greeks and Romans as history will play itself out up to the first century.
But, Paul is referring to another recent incident that you will find in Acts 2.1-41. Believers were gathered in the Temple on Shavuot (Pentecost) in obedience to the Lord (Exo 23.14-16). The Ruach ha Kodesh (Holy Spirit) comes upon the believers gathered there and this is also the day that God gave the Torah on Mt. Sinai, which testified about Yeshua (Psa 40.7). The unbelievers (key word) in Yeshua were amazed because these disciples were speaking in the known languages of the people gathered there.
They began to wonder what all this was about and what did it mean. Peter stands up and begins to give them a sermon that said that the tongues they were hearing was a sign from God that Yeshua was the Messiah and it was proof of it. He tells them that Yeshua was delivered up to them according to the predetermined plan of God and they murdered the very Messiah they have been waiting for, but God raised him from the dead as abtestimony against them.
The tongues prompted them to ask questions and Peter was able to preach to them in a language they all could understand. The people are shocked and they ask him what they should do. The tongues got the attention of the unbelievers there. Peter tells them to repent and be immersed in the name of Yeshua for the forgiveness of their sins and they will receive the immersion in the Ruach ha Kodesh (Holy Spirit).
So, there were many “mikva’ot” (immersion baths) at the southern end of the Temple (the very ones used can be seen today) and three thousand were saved and immersed. The people Isaiah was dealing with rejected the plain truth in their own language to repent and turn from their wicked ways. The Assyrians had already taken the northern tribes and will come to Jerusalem if they don’t repent. The Babylonians are also lurking in the shadows of future history.
But, the people were rejecting Isaiah’s message and God allowed the strange tongues of foreigners to eventually enter the land and nearly take the city of Jerusalem and Temple. The land was overrun by “people speaking in foreign tongues” but the Lord had mercy and the Assyrians were destroyed. But, not long after the people again turned from the Lord and rejected the message of Jeremiah and the other prophets and the Babylonians were allowed to run through the streets of Jerusalem, destroying the city and Temple and transporting the people to Babylon, where they heard many foreign tongues. They believed the prophets and their message after that happened. They could look back and see what happened.
In Acts 2, it is the same thing. The people as a corporate nation through their leaders had rejected the Messiah and the prophets concerning him. The teachers, priests and scribes were corrupt and by being so did not understand the hour of their visitation (Luke 19.39-44). They rejected the plain truth in their own language about Yeshua. As a result, they are hearing foreign tongues in their city and Temple again. The people needed to accept the words of the prophets concerning Yeshua and turn as a corporate nation to the mercy of God.
However, we know they didn’t, in fact, things get worse and by 70 AD God has had enough. The Romans and their auxiliaries, all speaking in foreign tongues to the unbeliever in Israel and Jerusalem, take the nation and destroy Jerusalem and the Temple. As a matter of history, the Babylonians and the Romans destroyed the Temple on the same day, nearly 700 years apart, the 9th of Av.
So, when Paul is saying that tongues is a sign for the unbeliever, he is saying that tongues are a warning of judgment to come for the unbeliever if they do not not heed the message of Yeshua and salvation. In their case spiritually, their “temples” would be destroyed and they will be lost, taken “out of the land” and deported to a foreign land called the Lake of Fire.