I don’t see what Darwin has to do with Secularism. To my mind, secularism begins with the notion of a secular government that essentially replaces Religious Authority with a Governmental Authority. For example, a government that allows things that God forbids is secular, while one that submits to God is not.
You can kind of see this in the Islamic world, which is one of the few places in which the culture is not wholely secularized. In those parts of the world, what the Qu’ran says guides how people live much moreso than what culture or government say. In fact, you could say that in a place like Saudi Arabia, Islam guides the government and the culture as well as economics. In a place like England or France, it’s the opposite – religion is steered by culture, economics, and government. The government can oftentimes contradict and even forbid common religious practices, or the culture can negate them, or the economy can make them difficult.
So given the history of Europe, I would suggest that the rise of the Nation over Christendom probably set us up for secularism. Before say the American or French Revolution, the government publicly chose a denomination. England was Anglican, France was Catholic, Spain was Catholic, and so on. After the revolutions, the idea of religion being at the center of a state fades – the Constitution allowed everyone to pick a denomination, the French tried to make up their own religion. But after that, the idea that the King or the State would publicly uphold a religion was seen as unenlightened. No longer was it really the case that the government would force religious morals on people – instead the government got the right to impose secular rules that at best don’t contradict religion. eventually, religion is brought to heel as governments decide they have the power to do anything and that no mere church should be able to declare otherwise.
So it starts, at least to me around the 1760s or so, though the Reformation had a part in setting the stage.
