Christian tombstones in wall

In Rome it is not uncommon for later buildings to upend and recycle items of earlier times.  Churches, in particular, utilize tombstones belonging to early Christian communities.  These Christians were initially buried outside of the city.  They started the trend of the modern date marking system for the tombs.  Symbols of the chi-rho, one of the earliest Christian symbols, were often carved on these in part because the idea of the cross scared early Roman/Jewish Christians (remember, it was a state penalty execution).  The chi-rho symbols were also used to indicate Caesar, which also meant "anointed one" and who was a symbol of winning/power.