Of the teacher should be plain
the Holy Bible

--1 Corinthians--

So likewise ye, except ye utter by the tongue words easy to be understood, how shall it be known what is spoken? for ye shall speak into the air. (14:9)

Yet in the church I had rather speak five words with my understanding, that [by my voice] I might teach others also, than ten thousand words in an [unknown] tongue. (14:19)

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