I Corinthians 14:22-25

Therefore tongues are for a sign, not to those who believe but to unbelievers; but prophesying is not for unbelievers but for those who believe. Therefore if the whole church comes together in one place, and all speak with tongues, and there come in those who are uninformed or unbelievers, will they not say that you are out of your mind? But if all prophesy, and an unbeliever or an uninformed person comes in, he is convinced by all, he is convicted by all. And thus the secrets of his heart are revealed; and so, falling down on his face, he will worship God and report that God is truly among you.