As I’ve argued in Religious Freedom in a Secular Age, “Secularism” is not one thing but several different things. There are different ways of being secular, different models of secularism, different ways of defining the relationship between religious groups and state authorities.
Secularism can be aggressive and authoritarian or it can be benign and postulate room for church-state cooperation.
The is a difference between the secularism of Britain, France, Turkey, India, Thailand, China, America, and Mexico.
While religious toleration is a practice that transcends empires - the Mongols and even the Ottomans were in varying degrees tolerant of other religions - secularism is very much a Christian invention.
Secularism was the Christian way of trying to manage religious diversity, defining the spaces where religion was not allowed to matter, and defining the places where religion was immune to government intervention.
For secularism to work, you need Christianity and expressive individualism rather than tribalized or sectarian identities.
It is precisely why “secularism” doesn’t work in countries that do not have those things. It explains why secularism is waning in Türkiye and India.
Secularism was introduced into Türkiye by Mustafa Kemal Atatürk as a way to modernize Türkiye and eliminate the power of the Sultan. The reason why current Türkiye President Recep Tayyip Erdogan is able to implement the ideals of his Islamicist party is for two reasons. First, Islam is not a “religion” in the sense of something you do for one hour a week and which exists between your ears, but it is more like a way of life or even a type of civilization. Second, secularism, where the state endeavors to remain neutral in matters of religion, is foreign and alien to the vast majority of the history of Türkiye.
Similarly, look at India. When the British colonized India, many of the native peoples were surprised to learn that they had a religion called “Hinduism.” It was the British who imposed their own distinction between religion/government/culture into India. Religion had always been part of politics in India, even Ghandhi’s independence movement was a largely Hindu enterprise, precisely why it created tensions with the Muslim minority. The Hindu nationalist policies of Narendra Modi are not an aberration, they are very much turning back the clock to the pre-British past when religion, whether among the Mughal or Vijayanagara Empires, was suffused into the culture.
Here’s the thing. Many of us take the separation of church and state as a self-evident brute fact of nature, like saying “grass is green,” “sky is blue,” and “secularism is normal.” But for most cultures for most of history, secularism has never been conceivable, because of the intersection between ethnicity, tribal identity, geography, and god(s).
To say to these people in these places, “Well, stop being religious and that will end all of your sectarian conflicts,” is like saying to people in the west, “Stop having individual rights and caring so much for personal liberty, embrace cultural homogeneity, and then you’ll cease having so many ridiculous rifts in your stupid culture wars.”
When it comes to religion, culture, and politics, we have to remember that foreign places are foreign, and nothing shows that more than our assumptions about secularism.
This exemplified by a question my 30+ daughter asked regarding the Coronation which she watched and found moving: i am surprised it is so religious? I mention her age because she has lived within the English culture (with a Christian mother) yet her culture is avowed secular and she had no idea of the religious or historical references within the ceremony. On my holidays to Egypt, Turkey there would have been complete congruence within society and culture of the faith worldview.
Secularism in Christian countries is a reaction to Christian sectarianism. The Enlightenment thinkers of France, US, etc. that pushed for a separation of Church and State were reacting either to things like the Thirty Years War, the Wars of Religion in France, tension with the Church and leadership under Elizabeth and the Stuarts, and more. While it may have been invented by people who were Christian culturally and religiously, it didn't stem from theological considerations, but rather from Ecclesial ones.
So while I am inclined to agree with you that it was articulated among Christians it doesn't necessarily flow from a Christian theological viewpoint, but a happenstance of context regarding ecclesial tensions.
I do appreciate your mention of the way we have privatized religion (in the Anglophone world especially) though, because it never has been the norm.