The mosaic decorations that were preserved from the Pontificate of Sixtus III, and that unfold in the central nave and triumphal arch, are some of the most significant examples of their kind. They mark the beginning of Christian narrative art on a large scale. The mosaics on the nave depict the history of the Hebrews ante sub legem, before and after God gave Moses the Tablets of the Law. The left side narrates the story of Abraham and Jacob, who, as precursors of Christ, represent the growth and evolution of the People of God. The opposite side depicts the People of God gaining the upper hand over their enemies under the guidance of Moses and Joshua.
The mosaics on the triumphal arch, which was the original front wall of the apsis, have been almost completely preserved in their entirety since circa 430. They depict the theme of Jesus’ childhood, that is, God’s revelation to his people through Christ, son of the Virgin Mary.