I always considered myself agnostic leaning atheist. I consider leftism to be antithetical towards religion. All religions have a class structure to them, do priests, rabbis, or imams not exist? And what about marxism, is karl marx not venerated by some leftists in a weird atheist sort of way?
I'm atheist but I don't fault anyone for practicing so long as it's not clashing with your leftism. I mean there's a long history of both theists and atheists on the left.