1 Corinthians 14:9

"So also ye, unless ye utter by the tongue, speech easy to be understood, ye will be speaking into the air:" i. e., calling to nobody, speaking unto no one. Thus every where he shows its unprofitableness. "But if it be unprofitable, why was it given?" saith one. So as to be useful to him that hath received it. But if it is to be so to others also, there must be added interpretation. Now this he saith, bringing them near to one another; that if a person himself have not the gift of interpretation, he may take unto him another that hath it, and make his own gift useful through him. Wherefore he every where points out its imperfection, that so he may bind them together. Any how, he that accounts it to be sufficient for itself, doth not so much commend it as disparage it, not suffering it to shine brightly by the interpretation. For excellent indeed and necessary is the gift, but it is so when it hath one to explain what is spoken. Since the finger too is a necessary thing, but when you separate it from the other members, it will not be equally useful: and the trumpet is necessary, but when it sounds at random, it is rather an annoyance. Yea, neither shall any art come to light, without matter subject to it; nor is matter put into shape, if no form be assigned to it. Suppose then the voice to be as the subject-matter, but the distinctness as that form, which not being present, there will be no use in the material.

You see that the barbarian philosophy professes deeds, not words. The apostle thus speaks: "So likewise ye, except ye utter by the tongue a word easy to be understood, how shall ye know what is spoken? for ye shall speak into the air. There are, it may be, so many kind of voices in the world, and none of them is without signification. Therefore if I know not the meaning of the voice, I shall be unto him that speaketh a barbarian, and he that speaketh shall be a barbarian unto me."

Lest they should say: "what relation does the example of the flute and trumpet have to us?", he says: if you too, by the gift of tongues, do not utter words that are intelligible, that is, clear, then you speak in vain and into the …

But the reason why it cannot be known what you are saying is that you will be speaking into the air, i.e., uselessly: "I do not box as one beating the air" (1 Cor 9:26).

Unless ye utter by the tongue - Which is miraculously given you. Words easy to be understood - By your hearers. Ye will speak to the air - A proverbial expression.