In this paragraph he goes on to show how vain a thing the ostentation of speaking unknown and unintelligible language must be. It was altogether unedifying and unprofitable (Co1 14:6): If I come to you speaking with tongues, what will it profit you, unless I speak to you by revelation, or by knowledge, or by prophesying, or by doctrine? It would signify nothing to utter any of these in an unknown tongue. An apostle, with all his furniture, could not edify, unless he spoke to the capacity of his hearers. New revelations, the most clear explications of old ones, the most instructive discourses in themselves, would be unprofitable in a language not understood. Nay, interpretations of scripture made in an unknown tongue would need to be interpreted over again, before they could be of any use.
I. He illustrates this by several allusions. 1. To a pipe and a harp playing always in one tone. Of what use can this be to those who are dancing? If there be no distinction of sounds, how should they order their steps or motions? Unintelligible language is like piping or harping without distinction of sounds: it gives no more direction how a man should order his conversation than a pipe with but one stop or a harp with but one string can direct a dancer how he should order his steps, Co1 14:7. 2. To a trumpet giving an uncertain sound, adēlon phōnēn, a sound not manifest; either not the proper sound for the purpose, or not distinct enough to be discerned from every other sound. If, instead of sounding on onset, it sounded a retreat, or sounded one knew not what, who would prepare for the battle? To talk in an unknown language in a Christian assembly is altogether as vain and to no purpose as for a trumpet to give no certain sound in the field or day of battle. The army in one case, and the congregation in the other, must be all in suspense, and at a perfect nonplus. To speak words that have no significancy to those who hear them is to leave them ignorant of what is spoken; it is speaking to the air, Co1 14:9. Words without a meaning can convey no notion nor instruction to the mind; and words not understood have no meaning with those who do not understand them: to talk to them in such language is to waste our breath. 3. He compares the speaking in an unknown tongue to the gibberish of barbarians. There are, as he says (Co1 14:10), many kinds of voices in the world, none of which is without its proper signification. This is true of the several languages spoken by different nations. All of them have their proper signification. Without this they would be phōnai aphōnoi - a voice, and no voice. For that is no language, nor can it answer the end of speaking, which has no meaning. But whatever proper signification the words of any language may have in themselves, and to those who understand them, they are perfect gibberish to men of another language, who understand them not. In this case, speaker and hearers are barbarians to each other (Co1 14:11), they talk and hear only sounds without sense; for this is to be a barbarian. For thus says the polite Ovid, when banished into Pontus,
Barbarus hic ego sum, quia non intelligor ulli,
I am a barbarian here, none understand me.
To speak in the church in an unknown tongue is to talk gibberish; it is to play the barbarian; it is to confound the audience, instead of instructing them; and for this reason is utterly vain and unprofitable.
II. Having thus established his point, in the two next verses he applies, 1. By advising them to be chiefly desirous of those gifts that were most for the church's edification, Co1 14:12. "Forasmuch as you are zealous of spiritual gifts, this way it will become commendable zeal, be zealous to edify the church, to promote Christian knowledge and practice, and covet those gifts most that will do the best service to men's souls." This is the great rule he gives, which, 2. He applies to the matter in hand, that, if they did speak a foreign language, they should beg of God the gift of interpreting it, Co1 14:13. That these were different gifts, see Co1 12:10. Those might speak and understand a foreign language who could not readily translate it into their own: and yet was this necessary to the church's edification; for the church must understand, that it might be edified, which yet it could not do till the foreign language was translated into its own. Let him therefore pray for the gift of interpreting what he speaks in an unknown tongue; or rather covet and ask of God the gift of interpreting than of speaking in a language that needs interpretation, this being most for the church's benefit, and therefore among the gifts that excel; vide Co1 14:12. Some understand it, "Let him pray so as to interpret what he utters in prayer in a language unintelligible without it." The sum is that they should perform all religious exercises in their assemblies so that all might join in them and profit by them. 3. He enforces this advice with a proper reason, that, if he prayed in an unknown tongue, his spirit might pray, that is, a spiritual gift might be exercised in prayer, or his own mind might be devoutly engaged, but his understanding would be unfruitful (Co1 14:14), that is, the sense and meaning of his words would be unfruitful, he would not be understood, nor therefore would others join with him in his devotions. Note, It should be the concern of such as pray in public to pray intelligibly, not in a foreign language, nor in a language that, if it be not foreign, is above the level of his audience. Language that is most obvious and easy to be understood is the most proper for public devotion and other religious exercises.

For if I pray in an unknown tongue - If my prayers are composed of sentences and sayings taken out of the prophets, etc., and in their own language - my spirit prayeth, my heart is engaged in the work, and my prayers answer all the purpose of prayers to myself; but my understanding is unfruitful to all others, because they do not understand my prayers, and I either do not or cannot interpret them. See the note on Co1 14:19.

For if I pray in an unknown tongue,.... In the Hebrew tongue, which the greatest part of the Jewish doctors insisted (a) upon should be only used in prayer; which notion might be borrowed from them, and now greatly prevailed in the church at Corinth; and the custom was used by such as had the gift of speaking that language, even though the body and bulk of the people understood it not:

my spirit prayeth; I pray with my breath vocally; or else with affection and devotion, understanding what I say myself, and so am edified; or rather with the gift of the Spirit bestowed on me:

but my understanding is unfruitful; that is, what I say with understanding to myself is unprofitable to others, not being understood by them.

(a) Vid. Trigland. de Sect. Kar. c. 10. p. 172, 173.

spirit--my higher being, the passive object of the Holy Spirit's operations, and the instrument of prayer in the unknown tongue, distinguished from the "understanding," the active instrument of thought and reasoning; which in this case must be "unfruitful" in edifying others, since the vehicle of expression is unintelligible to them. On the distinction of soul or mind and spirit, see Eph 4:23; Heb 4:12.