1 Corinthians 14:6-12
6 “But now, brothers and sisters, if I come to you speaking in tongues, how will I benefit you unless I speak to you either by way of revelation, or of knowledge, or of prophecy, or of teaching? 7 Yet even lifeless instruments, whether flute or harp, in producing a sound, if they do not produce a distinction in the tones, how will it be known what is played on the flute or on the harp? 8 For if the trumpet produces an indistinct sound, who will prepare himself for battle? 9 So you too, unless you produce intelligible speech by the tongue, how will it be known what is spoken? For you will just be talking to the air. 10 There are, perhaps, a great many kinds of languages in the world, and none is incapable of meaning. 11 So if I do not know the meaning of the language, I will be unintelligible to the one who speaks, and the one who speaks will be unintelligible to me. 12 So you too, since you are eager to possess spiritual gifts, strive to excel for the edification of the church.”
Paul does not want unintelligible speech to be present within the church services at Corinth. This is akin to simply speaking to the air. Instead, he wants the church services to benefit and edify believers unlike tongues which is an evangelistic tool meant for outside of the church.
Languages in the world: Paul is further defining the gift of tongues here. Notice that he defines the gift as relating to the languages of the world. But if the speaker of tongues does not know what he is saying then he ought not speak the language unless there is an interpreter. This suggests that believers who possessed the gift did not always understand what was being said.
