The mosaics are the work of Venetian and Sicilian masters (12C and 13C). They tell the story of the divine plan of universal salvation, from the creation of the world and of man up to the intervention of God. The coming of Christ incarnates the accomplishment of the Redemption (transept and lateral naves). The mission started by Christ continues through the birth of the Church and thanks to the holy men who follow his example (secondary apses). In the central apse the Christ Pantocrator reigns. The central nave, meanwhile, is dedicated to the depiction of the Old Testament.