The forms of ancient Christian, as of late Roman, art are psychologically, not metaphysically expressive; they are expressionistic but not revelatory. The wide open eyes of late Roman portraits express intensity of soul, spiritual tension, a life that is strongly emotional; but it is a life which is without any metaphysical background and as such has no inner relation to Christianity. It is in fact the product of conditions which obtained long before Christianity emerged. The tension which Christian doctrine resolves was already beginning to be felt in the Hellenistic age; though Christianity soon produced answers to the questions that troubled those times, the work of many generations was needed before those answers could be expressed in forms of art—these were by no means simultaneous with the enunciation of the doctrine itself.
Arnold Hauser The Social History of Art: Volume 1: From Prehistoric Times to the Middle Ages