The cathedra petri Reliquary and Papal Altar
One of the most important heirlooms of the basilica was a chair (Latin: cathedra) which was thought to have been the episcopal throne of Peter as the first bishop of Rome. The chair had been carefully preseved throughout the middle ages as a connection to St. Peter and a symbol of the line of bishops who had succeeded him as pontiffs. Bernini was commissioned to create an appropriate reliquary for the cathedra petri. It is a "speaking reliquary" since it tells by its shape what it's contents are. The chair is decorated on its back with an image of Christ as shepherd of his sheep. The chair seems to levitate beneath a gloria from which shoot forth rays of light accompanied by a host of angels and cherubs.
Flanking the cathedra petri on eithe side are the four "doctors of the church" whose theology became the basis for Roman Catholic beliefs and teachings throughout the history of the Church. The four figures are St. Ambrose, St. Athanasius, St. John Chrysostom, and St. Augustine.
The figure of St. Augustine gestures toward the levitating cathedra petri as if to verify that its original user is indeed the backbone of the papal office. The four church doctors, the cathedra petri, and the gloria form an appropriate context for the papal altar and a well-scaled focus for the western apse of the basilica.