I've only played Oblivion a little bit and, unfortunately, after playing Skyrim. But I can imagine in that game I would think he is divine because he has as many churches and priests as other divines in game. And of course they have a quest more or less confirming his divinity, if you believe that Martin need the "blood of a divine"...
However, when you look into his story and basically compare it to real life examples like Rome or even modern North Korea, it's not difficult to imagine him simply creating propaganda to force that image and I am assuming Bethesda is going to leave it up in the air like that to keep it interesting.