George Leo Haydock, on 1Cor 14:20

Tongues are for a sign, not to believers, but to unbelievers, according to what the law (under which he comprehends the prophet Isaias, xxviii. 11.) said: In other tongues, and other lips, I will speak to this people: and neither so will they hear me. St. Paul here gives the sense, rather than the words of the prophet, and expounds them of what happened particularly on the day of Pentecost, when the miraculous gift of tongues was designed to strike the unbelieving people with admiration, and to bring them afterwards to the true faith: but when he adds, that tongues are not for the believers, and that prophecies are not for the unbelievers, he cannot mean that tongues, used with discretion, may not also be profitable to believers, or that prophecies and instructions may not also be profitable to unbelievers, as well as to the believers; for this would be to contradict what he teaches in this chapter, and particularly (ver. 24.) where he says, that by prophecy the infidel is convinced, &c. St. Paul, whose design in all this chapter is to regulate the meetings, that they may be conducted with greater edification, and for the instruction, both of the Christians and unbelievers, thanks God, that he has the gift of tongues more than they, but says, that in the Church, or at such Church-meetings, he had rather, for the common edification of others, speak five words, &c. than ten thousand words, &c. and so he concludes, (ver. 39.) be zealous to prophesy, and forbid not to speak with tongues. (Witham)