“In 350, Pope Julius I declared that Christ’s birth would be celebrated on December 25. There is little doubt that he was trying to make it as painless as possible for pagan Romans (who remained a majority at that time) to convert to Christianity. The new religion went down a bit easier, knowing that their feasts would not be taken away from them.”
Just look at the calendar. The third and fourth centuries were a riot of controversy over the nature of the Person of Jesus Christ: God? Man? God-man union? God & man stuck together? So we had the big ecumenical councils to thrash out the definition.
Papal lightbulb goes on: Let’s institute a feast for the Universal Church to commemorate the Incarnation! I believe it was around 400 A.D.
The tradition tells us that He was conceived (not born) on March 25, which puts His birth in mid to late December. December 25th is as likely a date as any, and since it’s exactly nine months from March 25, it makes the point effectively.
In 336, at Rome - 14 years earlier. It was an anti-Arian feast; the liturgical expression of the Nicene teaching that the Son is “consubstantial with the Father.”
There are several reasons to be very doubtful of the explanation mentioned, despite its popularity on the Net: The feast is very unlikely to have been intended as a sop to converts from pre-Christian religions, if only because there were so many of these in Rome, with different feasts: the cult of Cybele was not that of Isis, important as they both were in Rome (& had been for a long time). Buddhists & Muslims aren’t Christian or Jewish - but neither are they the same religion. And neither were the pre-Christian religions of Rome. (Which is one reason the label “pagans” is apt to be very misleading.)
A further reason to be doubtful of that interpretation is that Christianity was extremely intolerant - that’s why there was so much vandalising of shrines & temples in Rome & elsewhere when the pre-Christian religions were outlawed in 391. People who destroy or defile or otherwise dishonour such places are not likely to pander to any residual affection for old ways that converts from those ways may have felt. Heretics were shown little sympathy - why would people whose past life was not in any sense Christian be treated differently ?
What is correct, is that the majority of the population of Rome was still following pre-Christian worships - Christians were more numerous in the Eastern half of the Empire; partly because Christianity was largely an urban religion; there were relatively few cities in the West.
The change of religions was slow even after 391. The Church had to Christianise time & space - this meant that the calendar, & the places of worship, with all that this implied for society at large, had to be changed. Individuals might change quickly - entire societies (& the religions which are part & parcel of them) are another matter entirely. Which is partly why even in an area like Southern Italy, close as it was to some of the main Christian centres in the West pre-Christian religion had not died out by 600.