It’s nice to find a Google query with more hits in Google Books than in Google itself.
“For pictorial representation is made use of in Churches for this reason; that such as are ignorant of letters may at least read by looking at the walls what they cannot read in books.”
Gregory refers to paintings, illustrations, sculpture and other visual representations used in Christian art to spread the the gospel in an era when only the clergy and the nobility were able to read.
But apart from being literate or illiterate, a picture is worth a thousand words.
Illustration: Triumph of Christianity