The religious aspects of Xmas have certainly changed over the years. Xmas used to be viewed as sinful & something unholy by most Americans. Nativity scenes even 150 years ago -- would get you thrown in jail & kicked out of the church. Today, things are much different ... well, maybe not in some parts of the nation. Say anything about separation of church & state & ... whew.
No matter how one worships or chooses not to worship, it is entirely a matter of conscience & conviction, a right protected by the Constitution. Yet, we would be greatly amiss to say that our nation has always celebrated Xmas. That is false. Xmas celebration is a relatively new phenomenon, only having been around for roughly 110 years or so, & even then with many different meanings than what we ascribe today.
For some more info on Xmas traditions & their history, here's another link.