I think that many people do not accurately quote what Paul said by not using his complete thought and sentence and therefore they do not properly comprehend what Paul meant when he said
"tongues are for a sign, not to those believing, but to the unbelieving;"
1 Cor 12:21-22
“By other tongues,
and by other lips,
I will speak to this people,
and not even thus will they hear Me,
says
the Lord.”
22So then, tongues are for a sign, not to those believing, but to the unbelieving; but prophecy
is not to the unbelieving, but to the believing.
To understand Paul's intent when he says "tongues are for a sign, not to those believing, but to the unbelieving" one should be careful to only understood in context of the prophecy of Isaiah 28 and not make up some strange application Paul never intended.
Isaiah 28:
10For precept
must be upon precept, precept upon precept; line upon line, line upon line; here a little,
and there a little:
11For with stammering lips and another tongue will he speak to this people.
12To whom he said, This
is the rest
wherewith ye may cause the weary to rest; and this
is the refreshing:
yet they would not hear.
13But the word of the LORD was unto them precept upon precept, precept upon precept; line upon line, line upon line; here a little,
and there a little; that they might go, and fall backward, and be broken, and snared, and taken.
(This is the judgment of the unbeliever who saw the manifestation of tongues and did not believe it)
Compare with such passages as John 12
36 When Jesus had said these things, he departed and hid himself from them.
37Though he had done so many signs before them, they still did not believe in him, 38so that the word spoken by the prophet Isaiah might be fulfilled:
“Lord, who has believed what he heard from us,
and to whom has the arm of the Lord been revealed?”
39Therefore they could not believe. For again Isaiah said,
40“He has blinded their eyes
and hardened their heart,
lest they see with their eyes,
and understand with their heart, and turn,
and I would heal them.
As you can grasp by reading both of these quotes from Isaiah that they are talking about the same thing, the fact that He did these supernatural signs and yet they still did not believe would result in their being driving further into darkness.
The gift of tongues was given to the church for more reasons than as a sign to the unbeliever but Paul is quoting the prophecy of Isaiah which specifically said that "yet they would not hear" (unbeliever)
The prophesy made it clear that this supernatural manifestation would not convince them, but it would be another nail in the coffin of the
unbeliever, making him so that he had no excuse for the blindness and hardness of heart that he would be given over to as a result of rejecting these supernatural signs.
IT WAS A SIGN to the Unbeliever in the sense that Isaiah said
11For with stammering lips and another tongue will he speak to this people.
12To whom he said, This
is the rest
wherewith ye may cause the weary to rest; and this
is the refreshing:
yet they would not hear.
You see the gift of tongues was not given to convince an unbeliever to be saved. It was a supernatural manifestation and YET THEY WOULD NOT HEAR = (UNBELIEVER). It was a sign to drive them further into darkness because of their unbelief.
Unbelievers will not be saved because they hear you speak in tongues. You misunderstood. But many do so. It is one of the most misunderstood statements of Paul and most misinterpreted.
The sign you are looking for that will cause an unbeliever to fall down at your knees and declare that God is in you of a truth and repent and be saved is the gift of Prophecy. You want to find a church that is operating in a gift of prophecy whereby an unbeliever can come visit and hear a word of prophecy that will expose the secrets of his heart and cause him to fall on his face in repentance.
So in the way that many think that Paul meant that tongues are sign for unbelievers and prophecy was for believers, they are thinking that tongues will make an unbeliever a believer but that is not what Paul is saying. He is saying that tongues are a sign for unbelievers to heap judgment upon them for their unbelief in the light of such a glorious manifestation of having done this great thing called speaking in tongues for the church and yet they still will not believe though they see it. But prophecy is a sign for the believers or (those that WILL BELIEVE) when they hear the prophetic word that convicts their hear and they repent.