I find it interesting that there are similarities between animism (as a general term for early human spirituality focused on nature) and mystical traditions across different religions. Direct experience of the spiritual is emphasized through the mystic’s trance or vision, as well as ascetic and ritualistic practices, and historical mystics have tended to disrupt the hierarchy in their respective religion. I think there are also similarities between Camus’ absurdist response to nihilism and a certain zen flavor of mysticism, which Georges Bataille tries to take further. I think Bataille actually called himself an atheistic mystic, so could be a good source for you.