1 Corinthians 14:21-23

Moffatt(i) 21 It is written in the Law, By men of alien tongues and by the lips of aliens I will speak to this People; but even so, they will not listen to me, saith the Lord. 22 Thus 'tongues' are intended as a sign, not for believers but for unbelievers; whereas prophesying is meant for believers, not for unbelievers. 23 Hence if at a gathering of the whole church everybody speaks with 'tongues,' and if outsiders or un believers come in, will they not say you are insane?