1 Corinthians 14:21-22

21It is written in the Law, “By men of strange tongues and by the lips of foreigners I will speak to this people, and not even then will they listen to Me,” says the Lord.

22Therefore, [unknown] tongues are [meant] for a [supernatural] sign, not to believers but to unbelievers [who might be receptive]; while prophecy [foretelling the future, speaking a new message from God to the people] is not for unbelievers but for believers.