Albert Barnes Commentary 1 Corinthians 14:9

"So also ye, unless ye utter by the tongue speech easy to understood, how shall it be known what is spoken? for ye will be speaking into the air." — 1 Corinthians 14:9 (ASV)

So likewise ye, etc. To apply the case: If you use a foreign language, how will it be known what is said, or of what use will it be, unless it is made intelligible by interpretation?

Utter by the tongue. That is, unless you speak.

Words easy to be understood. This means significant words, (see margin); words to which your hearers are accustomed.

For ye shall speak into the air. This means you will not speak so as to be understood; it will be just the same as if no one was present, and you spoke to the air.

We have a proverb that resembles this: "You may as well speak to the winds." This means you are speaking where you will not be understood, or where your words will have no effect.

It may be observed here that the practice of the papists aligns with what the apostle here condemns: worship conducted in a language not understood by the people.

Furthermore, much of this same kind of speaking occurs today when unintelligible terms are used, words are employed that are above the comprehension of the people, or doctrines are discussed that are unintelligible and regarded by them without interest.

All preaching should be plain, simple, perspicuous, and adapted to the capacity of the hearers.