Matthew Poole and continuators on 1 Corinthians 14:22

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

Wherefore tongues are for a sign, &c. that is, an eminent product of Divine Providence for the confirmation of the Truth of the Doctrine of the Gospel; signifying, that the Doctrine which was so delivered in every Nations language, must be from Heaven, from whence the first Ministers must have their power so to speak; yet doubtless, they were not only for a sign, being also a means, by which the knowledge of the Gospel was conveyed unto those who could not have understood what the Apostles and first Ministers of the Gospel said, had they not spoken to them in the Language of the Hearers.

When he saith, Prophesying serveth not for them that believe not: the meaning is, not only for them that believe not; for prophesying is certainly of use for them that believe not, for their conversion, as well as for them that believe for their edification.