1 Corinthians 14:22

'Wherefore (‘so that') tongues are for a sign, not to those who believe, but to the unbelieving. But prophesying is for a sign, not to the unbelieving, but to those who believe.'

The ‘wherefore' (so that) connects back to the previous verse. In the same way, he says, if you speak to the unbelieving in indecipherable tongues you will simply be confirming to them that God has no message for them, that judgment is at the gates. They will infer that they must be under judgment and that this God to Whom they have come to listen has nothing to say to them. Having come in to hear words from God it will be apparent to them that God is deliberately keeping His mysteries from them. And so their unbelief will be confirmed. Such will go away unbelieving. They will go away empty.

On the other hand, he says, if you prophesy then it will indicate that there is a real message from God for them and that it is for those willing to believe, and they will respond accordingly. Hope is offered. Belief will be the response. So tongues will only turn men away, while prophecy will draw them to belief.

'To those who believe.' That is, those who subsequently believe as a result of hearing the prophecy, those who are ready to believe, those who are potential believers, in direct contrast with those who go away confirmed in unbelief because of tongues. We can compare here those in Acts 17:32. Some mocked (strange tongues would have been good enough for them), others said, ‘we will hear you again on this matter'. As potential believers they must be spoken to in an understandable way.