The first Christian mosaics began to appear on the walls of the catacombs and tombs in the first centuries in Rome, but there were no large-scale buildings for the faith to express itself artistically. Early Christian basilicas used mosaic on the walls and floors, but the turning point for the art form came in the fifth and sixth centuries with the explosion of spiritual art in the Byzantine Empire.
The mosaics were made of tesserae: small, flat pieces of stone or glass that were cut and laid in plaster. The material was expensive and served to reflect the light and splendor of the church. The light from the windows illuminated the mosaics, and the inscriptions used lustrous gold tesserae so that the light would reflect and give the church a spiritual aura.
The mosaic images oriented the apse toward the Divine light in the East and helped connect the viewer to the heavenly realm by the body of Christ. The apse mosaic was an exemplary way to join the lofty church ideals with the illustrious Byzantine style.