1 Corinthians 14:21-22

In the law it is written, By men of strange tongues and by the lips of strangers will I speak unto this people; and not even thus will they hear me, saith the Lord. Wherefore tongues are for a sign, not to them that believe, but to the unbelieving: but prophesying is for a sign, not to the unbelieving, but to them that believe.