Silas's pretending to pray while looking for the objects his victims told him would hide the location of the keystone.
There's a brass strip running along the floor that delineates the Rose Line, a marker that shows the longitudinal axis that used to be considered the prime meridian (or zero longitude: now it resides in Greenwich, England).
The line ends at the peak of an Egyptian obelisk.
The Teacher told Silas that it laid beneath the Sign of the Rose, so he must be in the right place.
Meanwhile, Aringarosa's plane lands in Rome, and he's psyched to finally be on the offensive side of the battle.
It seems that Divine Intervention has given him the solution to the problem plaguing the Faith, and now everything's going to be a-okay.