“Pope Gregory the Great… reminded the [Christians] who were against all paintings that many members of the Church could neither read nor write, and that, for the purpose of teaching them, these images were as useful as the pictures in a picture-book are for children, ‘Painting can do for the illiterate what writing does for those who can read,’ he said…
“But it is clear that the type of art which was thus admitted was of a rather restricted kind. If Gregory’s purpose was to be served, the story had to be told as clearly and simply as possible, and anything that might divert from this main and sacred aim should be omitted.
“At first glance, such a picture looks rather stiff and rigid. There is nothing of the mastery of movement and expression which was the pride of Greek art… If the picture looks rather primitive to us, it must be because the artist wanted to be simple. The Egyptian ideas about the importance of clarity had returned with great force because of the stress which the Church laid on clarity. But the forms which the artists used in this new attempt were not the simple forms of primitive art, but the developed forms of Greek painting. Thus Christian art of the Middle Ages became a curious mixture of primitive and sophisticated methods.”
Ernst H. Gombrich, “Chapter 6: A Parting of Ways,” The Story of Art, 15th edition