1 Corinthians 14:11
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. 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.
If the meaning of a voice is not known, the one speaking and the one hearing are barbarians to each other, that is foreigners or strangers to one another. Speech that listeners cannot understand leaves speaker and hearer mutually strange, with no fellowship of meaning between them.
The principle is illustrated by lifeless instruments, for unless pipe or harp gives distinction in its sounds what is piped or harped cannot be recognized. It is illustrated further by war, for if the trumpet gives an uncertain sound no one is prepared for battle. It is illustrated also by speech itself, for words that are not easy to be understood are spoken into the air, even though there is no kind of voice in the world without meaning.
Therefore those who are zealous for spiritual gifts and spiritual speech must seek to excel to the edifying of the church. This excelling is done by uttering words easy to be understood, clear and distinct. Only understandable speech communicates, teaches, and builds up, while unclear speech does not communicate or edify.