How common was atheism/agnosticism/non-practicing believers in the Middle Ages compared to Westeros?

(Trying again because when I tried to do this post on July 3rd it got eaten by a browser crash and I was too busy to re-do it)

To begin with, it’s very hard to know how common social and cultural practices of ordinary people are in the Middle Ages because of the sources we’re dealing with. 

It’s especially difficult in this case, because Medieval European sources tend to conflate heresy, blasphemy, and atheism together – which means that you get very divergent opinions among modern scholars about whether you can use records of blasphemy to detect atheist thought or not. (In fact, the word atheism wasn’t used in the West until the 15th century, which means parsing terms gets very tricky indeed) By contrast, there’s actually a pretty rich literature of atheism in the Medieval Islamic world, albeit largely confined to an educated elite. 

For more on this, see:

Atheism in the Medieval Islamic and European World by Fatemeh Chehregosha Azinfar

The Oxford Handbooks on Medieval Christianity and Atheism.