> It is Christian in origin, albeit a minor local Christian tradition (developed in the US) rather than a major near-universal one.
It is Christian only in the sense that the people who celebrated it were Christian. The events that led to the festival were purely political in nature, and not necessarily wholesome either.
Thanksgiving, as a consistent national holiday formally observed every year, was established by Abraham Lincoln’s proclamation in 1863, which declared it a day for “Thanksgiving and Praise to our beneficent Father who dwelleth in the Heavens”. Clearly religious in nature, although no doubt Lincoln had political motivations as well - the two are not mutually exclusive. 1621 was in itself a one-off event; the fact that it was later cited as justification for a regular observance says more about those later times than about 1621, and how 1621 was presented at those later times (such as in 1863, but also various earlier times too-it was observed at the state/local level, and intermittently nationally too, before Lincoln made it a regular national observance) is more important than what really happened for the purpose of determining whether it should be classified as religious. Abraham Lincoln appears to have not believed in Christianity personally (his personal beliefs are obscure, but he may have been some kind of deist); but when he made that proclamation I think he was expecting most Americans to understand it in Christian terms and most Americans at the time in fact did.
Interesting. Money in the US has "In God We Trust," and it's not inherently religious. So I'm not sure the proclamation makes Thanksgiving religious. The origins might, though. I don't know enough about it to tell, and I'm not sure I care enough to research it. I don't really celebrate it as other than a day off from work though. Travel is a nightmare at that time, and, although this may sound like humblebragging, I try to be grateful at all times anyway.
> Money in the US has "In God We Trust," and it's not inherently religious
US currency in itself isn't inherently religious, but the decision to put that phrase on the currency was religious, and so is the decision to keep it there today. Now, it is not just religious, it is also very political, but the two are very often intertwined: it is a form of politicised religion, or religious politics.
> So I'm not sure the proclamation makes Thanksgiving religious. The origins might, though
I think to most Americans in the 19th century and earlier, the idea that Thanksgiving was a religious festival would have seemed obvious–it was about giving thanks to the deity, assuming a Judaeo-Christian conception of deity. Now, no denying that it became quite secularised through the course of the 20th century, and to many 21st Americans it is an entirely secular occasion, and if "thanksgiving" is anything more than an empty word, it is thanks directed at one's friends/family/colleagues/acquaintances/community/etc, maybe even at the cosmos, but not at God in whom one quite possibly doesn't believe.
As I said in my original comment, other religious festivals, such as Christmas, have also become highly secularised. But, even though many celebrate Christmas in an entirely secular way, people still remember its association with Christianity, which makes many non-Christians feel uncomfortable celebrating it even in a secular form. Thanksgiving was never so explicitly Christian, so Jews and Muslims and others feel more comfortable in celebrating it.
It is Christian only in the sense that the people who celebrated it were Christian. The events that led to the festival were purely political in nature, and not necessarily wholesome either.
1. https://www.theatlantic.com/ideas/archive/2019/11/thanksgivi...
2. https://www.salon.com/2016/11/23/thanksgiving-a-day-of-mourn...