1 Corinthians 14:18 (KJV)

Wherefore let him that speaketh in an unknown tongue pray that he may interpret.

For if I pray in an unknown tongue, my spirit prayeth, but my understanding is unfruitful.

What is it then? I will pray with the spirit, and I will pray with the understanding also: I will sing with the spirit, and I will sing with the understanding also.

Else when thou shalt bless with the spirit, how shall he that occupieth the room of the unlearned say Amen at thy giving of thanks, seeing he understandeth not what thou sayest?

For thou verily givest thanks well, but the other is not edified.

Yet in the church I had rather speak five words with my understanding, that by my voice I might teach others also, than ten thousand words in an unknown tongue.

Brethren, be not children in understanding: howbeit in malice be ye children, but in understanding be men.

In the law it is written, With men of other tongues and other lips will I speak unto this people; and yet for all that will they not hear me, saith the LORD.

Wherefore tongues are for a sign, not to them that believe, but to them that believe not: but prophesying serveth not for them that believe not, but for them which believe.

If therefore the whole church be come together into one place, and all speak with tongues, and there come in those that are unlearned, or unbelievers, will they not say that ye are mad?