I have a theory on this and it may be ridiculous. But I believe we are all agnostics to a degree whether we admit it or know it or not. That doesn't mean we don't subscribe to a certain religion or set of principles. However, as defined by Webster, :a person who holds the view that any ultimate reality (such as God) is unknown and probably
unknowable;
broadly :one who is not committed to believing in either the existence or the
nonexistence of God or a god*. To me, you can be a Bill Maher atheist or the Pope and there is at the very least a degree of doubt in either direction because most people realize 'it' has to be bigger than our understanding. Most people won't admit that, but at some point a person that adamantly disagrees with all faiths will occasionally ponder the idea that there is something out there and on the other hand I've heard most pastors admit they've doubted as well.
This isn't me answering this question but just a thought on the question since we typically end up at some point bringing up 'Can atheists and agnostics be Masons' etc.
*"agnostic."
Merriam-Webster.com. 2017.
https://www.merriam-webster.com (13 Oct 2017)