A prosperous Asiatic sun religion dwelt on the Vatican hill before the Popes commandeered it for Christianity. Mithras was an Aryan sun god, called by the Romans “the Unconquerable Sun”. The reform of the Persian religion by Zoroaster (Zarathustra) had put the ethical deity Ahuramazda so high above the old nature gods that he was practically the one god. But Mithras stole upward, as gods do, and Persian kings of the fifth century BC put him on a level with Ahuramazda. The Persians conquered and blended with Babylon, and Mithras rose to the supreme position and became an intensely ethical deity. He was, like Aten and Christ, the sun of the world. He sacrificed the pleasures of life, like Christ, but unlike Zeus.

Mithraism spread rapidly, was respected, and was strikingly like Christianity. During the third and fourth centuries AD, Mithras had become the most important solar god in the Roman Empire. Drastic asceticism and purity were demanded of his worshippers. They were baptized in blood. They practiced the most severe austerities and fasts. They had a communion supper of bread and wine. They worshiped Mithras in underground temples, artificial caves called grottos, which blazed with the light of candles and reeked with incense.

They celebrated the epiphany of this god, saviour of the world, on 25 December. Aurelian adopted the 25 December in 274 AD as the day to celebrate Natalis Solis Invicti, the birthday of Sol Invictus. As that day approached, near midnight of the 24th, Christians might see the devotees of Mithras going to their temple on the Vatican, and at midnight it would shine with joy and light. The saviour of the world was born. He had been born in a cave, like so many other sun-gods, and some of the apocryphal gospels put the birth of Christ in a cave. He had had no earthly father.

F Cumont, the great authority on Mithras, who it is now fashionable to disparage, collected for us details about the Persian religion, and more than one of the Christian Fathers refers to the similarity of the two religions. Mithras had had 25 December as his birthday for ages. He was eternal—the unconquered and unconquerable sun—the sun god as a spiritual god, with light as his emblem and honesty his supreme command. What could the Christians do? Nothing, until Constantine. Then they took 25 December, and Mithraic garb, customs and ritual, and so zealously dissolved the Mithraic religion into Christianity that only scholars know anything about it.