The painting depicts Saint Basil the Great (ca. 330–379) saying mass in the presence of the Emperor Valens, a heretical Arian. Surrounded by priests, Basil receives the wine for consecration. Valens's required gifts of bread are presented by figures at the left while to the right the emperor swoons, moved by the solemnity of the mass. This is a highly finished proposal, or modello, for Subleyras's most important commission: the full-scale design for a mosaic altarpiece for Saint Peter's in Rome. The painter seems to have retained the modello for himself. The frame is original to the picture.