It is a private oratory of Trinitarian bishops dating from the turn of the 6th century.

The tiny cruciform chapel is currently dedicated to Saint Andrew, although the original dedication was to the Saviour, as evidenced by a lunette over the vestibule door representing a beardless Christ treading on the beasts, dressed as a general or victorious Emperor.

The lower parts of the walls are lined with marble slabs, while the rest of the interior used to be covered with rich, tapestry-like mosaics, as the vault still is.

Some parts of these survive, while others were substituted with tempera paintings by Luca Longhi in the 16th century.

According to the ICOMOS evaluation of this World Heritage Site, "the significance of this property is the fact that it is the only Early Christian private oratory that has survived to the present day.