Roma CI was one of many Known Worlds where the Roman Empire never fell. There, Constantine worshiped Mithras, who led him into victory in battle; Mithraism became the official religion, and no other faiths were tolerated.
When Marcus Americanius Scriptor first visited Roma CI, Emperor Flavius Venerarius was the absolute ruler. Scriptor originally believed it was the same as Roma LVIII, which was alike in all ways except that Constantine instead worshiped Christ. (PROSE: Warlords of Utopia)