OK, I think I found it. It is from Jerome’s letter 52 to Nepotian:
When teaching in church seek to call forth not plaudits but groans. Let the tears of your hearers be your glory. A presbyter’s words ought to be seasoned by his reading of scripture. Be not a declaimer or a ranter, one who gabbles without rhyme or reason; but show yourself skilled in the deep things and versed in the mysteries of God. To mouth your words and by your quickness of utterance astonish the unlettered crowd is a mark of ignorance. Assurance often explains that of which it knows nothing; and when it has convinced others imposes on itself. My teacher, Gregory of Nazianzus, when I once asked him to explain Luke’s phrase σάββατον δευτερόπρωτον, that is the second-first Sabbath, playfully evaded my request saying: I will tell you about it in church, and there, when all the people applaud me, you will be forced against your will to know what you do not know at all. For, if you alone remain silent, every one will put you down for a fool. There is nothing so easy as by sheer volubility to deceive a common crowd or an uneducated congregation: such most admire what they fail to understand. Hear Marcus Tullius, the subject of that noble eulogy: You would have been the first of orators but for Demosthenes: he would have been the only one but for you. Hear what in his speech for Quintus Gallius he has to say about unskilled speakers and popular applause and then you will not be the sport of such illusions. What I am telling you, said he, is a recent experience of my own. One who has the name of a poet and a man of culture has written a book entitled Conversations of Poets and Philosophers. In this he represents Euripides as conversing with Menander and Socrates with Epicurus— men whose lives we know to be separated not by years but by centuries. Nevertheless he calls forth limitless applause and endless acclamations. For the theatre contains many who belong to the same school as he: like him they have never learned letters.
You can see from this that the context has been distorted. Jerome is telling Nepotian *not *to act like this, and he explicitly says that St. Gregory was joking with him, threatening to embarrass him by giving a fake explanation of the difficult passage and making everyone applaud. Certainly the text reflects a certain elitism, but no intent to deceive. If you read the letter as a whole, you’ll see that Jerome is holding up a very high standard for clerical behavior. 18th-century rationalists are not to be trusted when they report on early Christians!