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We know he was Saint George, the Dragonslayer... or at least the original person that later Christianity renamed Saint George. Also his father might be the origin of the Abel (his father) and Cain myth. Just with the Emperor being the one cursed to live forever, with him killing his uncle cain when he found out. Though the Emperor confirmed the first murder was long before his birth and near after when humans first evolved from apes. He's suspected of being Hammurabi, Alexander the Great, Julius Caesar, Charlemagne, Napoleon and Abraham Lincoln. Of these Alexander the Great has the most backing, especially in older lore but Horus Herecy does have his ship named after Alexander's horse. However we can rule out him being Genghis Khan, as that man is confirmed to have later become a demon prince of Khorne From mythology, others suspect he was the origin of Gilgamesh, Hercules, and Merlin. Ironically, while he hates religion, many suspect he could have been at least one of several Messiah figures of Moses, Jesus, and Mohammed are the most common but of the three Jesus is the most common. Probably because he's the most known and least controversial... and A lot of old 80s and 90s lore implied he was jesus.

Good stuff

jesus christ, 5 minutes of my life i wont get back

No, he was not Jesus. This can be debunked quite easily: Ollanius Pius. Ollanius Pius was a close friend to the emperor(being another perpetual), he was also catholic, and knew the emperor was not the God nor a god. It's far more likely that the Emperor was Judas.