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Christmas was not among the earliest festivals of the Church. Irenaeus and Tertullian omit it from their lists of feasts.
so.. Happy Roman Saturnalia and Scandinavian / Teutonic Yule.
The largest pagan religious cult which fostered the celebration of December 25 as a holiday . . . was the pagan sun- worship, Mithraism. The Mithraists also observed Sun-day and kept sacred the 25th of December as the birthday of the Sun.

Mithraism is an ancient roman religion from the 1st century BCE. It flourished in the first few centuries CE by which time it had many features in common with Christianity (as did multiple religions and cults of the era) including the motif of a crucified-and-resurrected god-man who comes to bring salvation from sin, and the primacy of 12 followers. Mithraism and Judaism merged and became the Christianity that we know today. Jesus, son of the Hebrew sky God, and Mithras, son of Ormuzd are both retellings of the same myth. The rituals of Christianity coincide with the earlier rituals of Mithraism, including the Eucharist and the Communion in great detail. The religious language used by Mithraism became the language used by Christians.
The idea of a sacrificed saviour is Mithraist, so is the symbolism of bulls, rams, sheep, the blood of a transformed saviour washing away sins and granting eternal life, the 7 sacraments, the banishing of an evil host from heaven, apocalyptic end of time when God/Ormuzd sends the wicked to hell and establishes peace. The archaeological picture is clear that Mithraism and Christianity developed out of the same religious culture but each going in (slightly) different directions. Neither religion had a “starting point” defined by a single individual or founder in the first century, but they are continuums of developing belief from the 1st and 2nd century BCE.

The worship of Mithra was first recognized by Emperor Aurelian and he instituted the cult of “Sol Invictus” or the Invincible Sun. Emperor Diocletian also a worshipper of Mithra, the Sun God, burned much of the Christian scriptures in 307 A.D

((The transformation of Mithras into a Bull or Ram which preceded the eating of his flesh and blood directly parallels the Christians Jesus’ death and rebirth, his statement that his disciples should eat and drink his flesh and blood to wash away sin and gain eternal life…… you see.. the jewish version included cannibalism.. at least the Mithraists transformed the human into an animal….))