During the reign of Marcus Aurelius (r.161-180), the Roman Empire started to experience increased pressure on its frontiers. Germanic tribes started to organize themselves better and in the East, the Parthians were replaced by the Sasanian Empire, which was more aggressive than its predecessors had been. The Roman emperors took countermeasures and tried to gain divine support by persecuting religious minorities, like the Manichaeans, the Jews, and the Christians. By ancient standards, this was a logical decision: the fact that they did not worship the gods of the state, offered sufficient explanation for Roman military defeats.
The Persecutions were very serious, and you do not need to be a Christian to abhor from the state’s violence against its own citizens. It is always fitting and proper to commemorate the slaughtered innocents. For this reason, pope Benedict XIV (r.1740-1758) dedicated the Colosseum to the memory of the Christian martyrs killed in the arena. The problem is that this is probably not a historical fact.
There are several texts about the martyrdom of Roman Christians. We know that Sebastian was executed on the Palatine and that Agnes suffered in the Stadium of Domitian. But no one is mentioned as being killed in the Flavian Amphitheater, as the execution theater was officially called. In the Acts of Justin, Chariton, Charito, Euelpistus, Hierax, Paeon, Liberian, and their Company, we read that these people were led “to the usual place”, which has been taken as a reference to the Colosseum, because we do not know which alternatives exist. However, this is poor evidence, and the fact that the Colosseum is not mentioned in Medieval catalogs of martyr shrines can mean only two things: if Christians were killed in the Colosseum, it was forgotten in the Middle Ages, or there were no Christians killed over there.
Of course, this does not mean that Benedict’s cross must be removed. It is part of the history of the Colosseum, and besides, it is never wrong to spend a thought about the terrible things that happened on this terrible place.