1 Corinthians 14:2

For he that speaketh in an unknown tongue
Or with tongues, as some copies and the Ethiopic version read: Dr. Lightfoot thinks, that the Hebrew tongue, which was become a dead language, and understood but by few, is here meant, and that not without reason; seeing the public prayers, preaching, and singing of psalms among the Jews, were in this languages ^F24; in imitation of whom, such ministers, who had the gift of speaking this language, read the Scriptures, preached, prayed, and sung psalms in it, which were no ways to the edification of the people, who understood it not; upon which account the apostle recommends prophesying, praying, and singing, in a language that was understood: otherwise he

speaketh not unto men;
to the understanding, profit, and edification of men: but unto God: to his praise and glory, and he only knowing, who knows all languages, and every word in the tongue what is said; excepting himself, unless there should be any present capable of interpreting:

for no man understandeth him:
or "heareth him": that is, hears him, so as to understand him; he may hear a sound, but he cannot tell the meaning of it, and so it is of no use and advantage to him:

howbeit in the Spirit he speaketh mysteries;
though under the influence and by the extraordinary gift of the Spirit he has, and to his own Spirit and understanding, and with great affection and devotion within himself, he speaks of the deep things of God, and the mysteries of his grace, the most glorious truths of the Gospel, yet the meaning of his voice and words not being known, he is a barbarian to them that hear him; and though what he delivers are truths of the greatest importance, they are a mere jargon to others, being unintelligible.