S3E11: Is Michael Satan?
Full disclosure: I come up with shirt theories all the time - bring me great joy in fact, however, one of my favourite things about the show is Michael's character arc.
He went from being a demon who took a lot of pleasure in his job to caring deeply about the four he was meant to be torturing to caring about the well-being of all lovely humans (granted there is an argument that this last bit doesnt really exist, like gravity as a force, but rather is an incidental by-product of his love for team cockroach, but I believe he's genuinely batting for humanity as a whole). He's railing against the system, against powers far greater than he for a noble cause.
We've seen such rebellious divinity explored in literature, famously Paradise Lost, and I can't help but think Michael is a recharacterisation of Satan.
(Coincidentally, i also wonder if Michael could have submitted himself as evidence that every thing in the after life isn't at all kosher. If he was capable of such a drastic change, might there not be some sort of Anti-Michael who set about perverting a good place neighbourhood?)