I think to a good extent you can say Catholicism grew by absorbing other religions. It started with Judaism, but then continued to subsume other pagan religions. Christmas itself has pagan roots.
So in European context, think of a religion believing in Norse and Greek gods as well as Christian God simultaneously in a single fabric.
I was under the impression that the "Calculation hypothesis" (Easter + 9 months) was the most likely:
* https://en.wikipedia.org/wiki/Christmas#Calculation_hypothes...
If you're going for 'historical accuracy', that'd be an argument, but that may not have been much of a consideration 2000 years ago.
> Christmas is definitely based on pagan traditions though, which you can see in the traditions of bringing a tree into the house and decorating it.
Some early mentions around 700 AD, but didn't really become a big thing until the 1500s and Protestantism it seems. That'd be fairly far removed from co-opting Pagan traditions, which were probably not practiced for a few hundred years at that point:
* https://en.wikipedia.org/wiki/Christmas_tree#Religious_issue...
Though these are on the secular side of things, as most liturgies don't incorporate them. This was mainly for Europeans, AFAICT.
Not sure how many Christians in tropical countries are doing the tree thing. Though with globalization and mass media, Western practices may be more spreading more places in recent years/decades.