Mithraism
An almost extinct religion devoted to the worship of Mithras. Ikkulu used them to hide many of her secrets, which is somewhat puzzling since they do not allow women to partake of their mysteries.
There is a small sect of them hidden away in Rome. Eorl, Thalia, Wirshin, Jean-Claude, and Gur encountered them after escaping the Tiny Dungeon. Ali al-Hajjad was already ensconced with them for some reason.
There are many similarities between this religion and Christianity, including their use of crosses or crucifixes in their iconography and the belief that both Mithras and Jesus Christ ascended into Heaven. Further similarities include how each of them was born from a virgin on December the 25th, how both of their followers identified with each of them by eating a sacred meal of bread and wine, and how their devotees were baptized in water.