I’ve been trying for a while to figure out when atheism became mainstream in the West. Sometimes I answer, “Around the end of the English Civil War,” (1650) and sometimes I answer, “Late 1980s/early 1990s.”
Medieval Europeans seem to have been pretty solidly Christian–probably about as Christian as modern Muslims are Muslim.
Modern Westerners are highly atheistic–even many of the “Christians”. So what happened?
I speculate that the upper classes in France, Britain, and the Colonies (and probably that-which-would-become-Germany and a few other places I’m less familiar with, like the Netherlands,) were largely atheistic by the 1700s. Look at the writings of the Enlightenment philosophers, the behavior of the French nobility, the English distrust of any kind of religious “enthusiasm,” German bishops actively encouraging Jewish settlement in their cities and attempting to protect the Jews from angry peasant mobs, various laws outlawing or greatly limiting religious power passed during the French Revolution, the deism of the Founding Fathers, etc.
By contrast, the lower classes in NW Europe and especially America retained their belief for far longer–a few isolated pockets of belief surviving even into the present. For example, see the Pilgrims, the Counter-Revolution in the Vendee, maybe German peasants, televangelists in the 80s, blue laws, and Appalachian snake handlers in the ’50s, etc.
So how did that happen? I propose that the upper class and lower class followed different evolutionary trajectories (due to different conditions), with strong religiosity basically already selected out by the 1700s, meaning the relevant selection period is roughly 500-1700, not post-1700s.
During this time, the dominant religion was Catholicism, and Catholicism has generally forbade its priests, monks, nuns, etc., from getting married and having children since somewhere abouts the 300s or 400s. (With varying levels of success.)
Who got to be an official member of the Church hierarchy? Members of the upper class. Peasants couldn’t afford to do jobs that didn’t involve growing food, and upper class people weren’t going to accept peasants as religious authorities with power over their eternal souls, anyway. Many (perhaps most) of the people who joined the church were compelled at least in part by economic necessity–lack of arable land and strict inheritance laws meant that a family’s younger sons and daughters would not have the resources for marriage and family formation, anyway, and so these excess children were shunted off to monasteries.
There was another option for younger sons: the army. (Not such a good option for younger daughters.) Folks in the army probably did have children; you can imagine the details.
So we can imagine that, given the option between the army and the Church, those among the upper class with more devote inclinations probably chose the Church. And given a few hundred years of your most devote people leaving no children (and little genetic inflow from the lower classes,) the net result would be a general decrease in the % of genes in your population that contribute to a highly religious outlook.
(This assumes, of course, that religiosity can be selected for. I assume it can.)
Since the lower classes cannot join the Church, we should see much more religiosity among them. (Other factors affected the lower classes, just not this one.) If anything, one might speculate that religiosity may have increased reproductive success for the lower classes, where it could have inspired family-friendly values like honesty, hard work, fidelity, not being a drunkard, etc. A hard-working, moderately devout young man or woman may have been seen as a better potential spouse by the folks arranging marriages than a non-devout person.
Religiosity probably persisted in the US for longer than in Europe because:
1. More religious people tended to move from Europe to America, leaving Europe less religious and America more;
2. The beneficial effects of being a devout person who could raise lots of children were enhanced by the availability of abundant natural resources, allowing these people to raise even more children. NW Europe has had very little new land opened up in the past thousand years, limiting everybody’s expansion. The European lower classes historically did not reproduce themselves (horrific levels of disease and malnutrition will do that to you), being gradually replaced by downwardly-mobile upper classes. (There are probably regions in which the lower classes did survive, of course;)
3. By the time we’re talking about America, we’re talking about Protestant denominations rather than Catholicism, and Protestants generally allow their clergy to marry.