The Mass of Saint Gregory is a subject in Catholic art which first appears in the late Middle Ages and was still found in the Counter-Reformation. Pope Gregory I (c. 540–604) is shown saying Mass just as a vision of Christ as the Man of Sorrows has appeared on the altar in front of him, in response to the Pope's prayers for a sign to convince a doubter of the doctrine of transubstantiation. Source: Wikipedia (en)