I Corinthians 14:9-11

So likewise you, unless you utter by the tongue words easy to understand, how will it be known what is spoken? For you will be speaking into the air. There are, it may be, so many kinds of languages in the world, and none of them is without significance. Therefore, if I do not know the meaning of the language, I shall be a foreigner to him who speaks, and he who speaks will be a foreigner to me.