In the beginning of DOOM, you start as DOOM guy and are lying on a big stone tomb. He has the strength to rip the chains that are holding him down and crush the skull of a possessed. This is before getting the armor. You then get the armor and you are the only one that can get it to start. Some scripture kind of things later in the game refer to a doom slayer and the game calls you a doom marine. This pretty much points to you being a demon or at least something other-worldly.