Therefore adherents of a religion are also not implicit in extremism, right?

That's literally laughable. Religion is a conscious choice to believe in something for which there is no evidence (which is colloquially known as "faith"). Allowing evidence to provide an understanding of how the natural world works is not the same as choosing to be a part of a community that is not based on reality.

It seems that we’re mostly in agreement that it’s the broad category of humans who are culpable

Correct. However, we differ in our definition of extremism, which I define as intolerance of others, willful ignorance of the natural world, and desire to restrict the rights of others based on their interpretation of Bronze Age manuscripts.