1 Corinthians 14:2

For he that speaketh in an unknown tongue; by a tongue (for unkown is not in the Greek, but necessarily added by our translators, for he speaketh of such a language) he meaneth a language not known to all, or at least not to the most of them that hear him. It may be asked, what unknown language the apostle here meaneth? Shall we think that any pastors or teachers in the church of Corinth were so vain, as to preach in the Arabic, Scythian, or Parthian language to a people who understood only the Greek? Our learned Lightfoot thinks this not probable, and that if any had been so vain for ostentation, the apostle would rather have chid them for suffering such an abuse, and have forbidden such further practice, than have given direction, than if any so spake he should interpret, as he doth, 1 Corinthians 14:5. He rather thinks, therefore, that the apostle meaneth the Hebrew tongue; the use of which, though it was by this time much lost through the Jews mixture with other nations, yet was restored in a great measure to the guides of churches, for their better understanding the Scriptures of the Old Testament; and continued amongst the Jews in their reading of the law in the synagogues. Now there being many Jews in this church, and the service of God being ordinarily in the Jewish synagogues performed in that language, it is very probable, that some of these Jews that were Christianized (to show their skill) might, when they spake to the whole church of Corinth, use to speak in Hebrew, though few or none understood that language. The apostle saith, he that did so, spake not unto men, that is, not to those men who did not understand that language, not to the generality of his hearers, though possibly here and there some might understand him, but unto God, who being the Author of all languages, must necessarily know the significancy of all words in them: for (he saith) scarce any man understood him. Howbeit in the spirit he speaketh mysteries; howbeit he may speak mysterious things to himself, and to the understanding of his own soul and spirit. Others think that it was possible, that some who thus spake, being but the instruments of the Holy Spirit, might not themselves understand all which they said; but that is hardly probable.