I find it fascinating that, unlike today, where every religion thinks that their own gods exist and others don't, the Romans believed that all other gods existed alongside their own.

When the Romans were conquering Britannia, they were attacking a small river island. There they encountered a group of Druids. As the Romans cut them down they realized that the Druids were actually sacrificing themselves at their hands in some unknown ritual. The Romans saw this as a bad omen since they had just inadvertently participated in some foreign god's ritual. Shortly afterwards, Boudicca started her campaign against the Romans.