John Chrysostom, on 1Cor 14:22

Having shown then even from the prophecy, that the sign in question is not of great use, he adds, "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."

Great in this place is the difficulty which one seems to find arising from what is said. For if tongues are for a sign to them that believe not, how saith he, if they that believe not should see you speaking with tongues, they will say that "ye are mad?" And if prophecy be "not for the unbelieving, but for them that believe," how shall also the unbelievers gain thereby?

And not only this, but also after this another question hence springs up: since the tongue will appear on the contrary greater than the prophecy. For if the tongues are for a sign to the unbelieving, but prophecy to them that believe, that which draws in aliens and makes of the household, is greater than that which regulates those of the household. What then is the meaning of that expression? Nothing difficult nor obscure, nor contrary to what went before, but rather very agreeable to it, if we give heed: viz., that prophecy is suitable to both, but then tongue not so. Wherefore having said of the tongue, "it is for a sign," he adds, "not to them that believe, but to the unbelievers," and to them "for a sign," i. e., for astonishment, not so much for instruction.

"But in the case of prophecy too," saith some one, "he did the very same thing, saying, 'but prophesying serveth not for the unbelieving, but for them which believe.' For the believer hath no need to see a sign, but requires only teaching and catechizing. How then sayest thou," saith he," that prophecy is of use to both, when Paul saith 'not to the unbelieving, but to them which believe?'" If thou wilt accurately examine, thou wilt understand what is said. For he said not, "prophecy is not useful to them unbelieving," but, "is not for a sign," as the tongue," i.e., a mere sign without profit: nor is the tongue any way useful to believers; for its only work is to astonish and to confound; the word "sign" bring one of those which may be taken two ways.