Richter: You said they'd be killed publicly.
Robert Langdon: Yes, revenge. For La Purga.
Richter: La Purga?
Robert Langdon: Oh geez, you guys don't even read your own history do you? 1668, the church kidnapped four Illuminati scientists and branded each one of them on the chest with the symbol of the cross. To 'purge' them of their sins and they executed them, threw their bodies in the street as a warning to others to stop questioning church ruling on scientific matters. They radicalized them. The Purga created a darker, more violent Illuminati, one bent on... On retribution.
When the antimatter explodes in the helicopter, the the shock-wave blasts off part of the roof inside St. Peter's Basilica, above the alter, obliterating everything below. However, in the scene where the Camerlengo self immolates himself at the altar close to the end of the movie, there is no evidence of the shock wave earlier, the candles are burning, oil lamps are lit and alter is intact.
During the making of the film, Tom Hanks personally escorted a couple whose wedding party coincided with scheduled filming at the Pantheon.