The 40k version of Martin "fuck that pope guy" Luther (not that one) and John the Baptist, Sebastian Thor was a super-charismatic preacher who arose during the Age of Apostasy and was responsible for leading the rebellion that saw the death of Goge Vandire, the reformation of the Ecclesiarchy to prevent such an individual from ever appearing again, and the creation of the Sisters of Battle.
Perhaps one of the most widely recognized Saints of the Imperial Cult. A significant minority of Imperials believe he was an avatar of the God-Emperor himself, to the point that a faction of the Inquisition that pursues a goal of finding a way to reincarnate the Emperor is known as the Thorians.
Thor's background is something of an enigma to the already spotty writings of the Ecclesiarchy and the Administratum; all that is known about him prior to his ascension into sainthood is that he lived on the planet of Dimmamar as merely a humble priest, preaching the Emperor's word as much as he could, never wanting for much and never coveting power, only bringing people together through sheer passion and faithfulness to the Imperial Cult. But the thing was, Thor lived during the Reign of Blood, and at that time the word he'd grown up preaching had been quickly warped and twisted so that it was unrecognizable as the Imperial religion. He was an adherent to the Confederation of Light, a more old-school "died for your sins" type of Imperial Cult, and finally found he could stand no more of Vandire's excesses, driving him to begin speaking out about the corruption taking hold at the heart of the Ecclesiarchy.
Normally, this would've gotten somebody shot dead in this time, but Thor was charismatic, and even more useful to him, he was popular. Vandire's agents could never quite get to him due to his sea of followers, and the few assassins that could get to him would come to find they had more in common with him than they ever did with Vandire, and would be quickly converted, leading to assassins converting assassins converting assassins... assuming they didn't just euthanize the ones that were too close to Vandire.
This naturally pissed off Vandire something fierce, and he sent an army to hate-fuck Dimmamar in half with guns, but they got plunged into a freak warp storm (which is still around today, as the Storm of the Emperor's Wrath) that made it clear to anyone watching that the Emperor's humble son was the winning team, which only made his following explode in size further. Finally, he began outwardly protesting and demanding reform from Vandire's ways and more towards Confederation of Light-thinking. He even began picking up followers from the Space Marines and Mechanicus, which eventually led to Vandire finally just openly declaring war on the Imperium at large, and ultimately his downfall at the hands of Alicia Dominica of the Brides of the Emperor, ending the Reign of Blood for good.
Of course, that meant the job of Ecclesiarch was now very much up for grabs, and he was naturally placed within that role, and he went about unfucking all the things Vandire fucked up within the Imperium, creating a separation between Ecclesiarchy and Administratum, issuing the Decree Passive, and restoring the High Lords as they previously were to their normal positions. Of course, he also decided to loophole his way through the Decree Passive by reorganizing the Brides of the Emperor into the Adepta Sororitas, specifically creating the Synod Ministra on the Cardinal World of Ophelia VII so that the Ecclesiarchy wouldn't be manipulated by the Administratum or another Ministorum ever again, and the subsequent founding of three Orders Militant of the Sororitas have only ensured that if something like this ever happens again, Thor's safeguards will come down on the dipshit who even tries fucking around like a massive beartrap.
Thor kept doing this until he was 112, only visiting Terra once since the liberation from Vandire's reign in that time.
At first, the High Lords offered the title of Ecclesiarch, but to their shock, he refused. They got butthurt and declared Sebastian Thor a traitor, and had him brought to Terra to face trial.
The Captain-General of the Custodes ruled in Thor's favour and aquitted him, all Terrans celebrating afterwards.
The High Lords, again, offered Thor the title of Ecclesiarch, but he, again, refused, and they, again, got pissed.
However, the Captain-General whispered something to Thor ( most probably a threat the likes of "Don't you fuckn' make me spend more time with these assholes than I already have to" ) and he accepted, under some conditions: he could go where he wanted, the High Lords supported him at all times, and he could make changes to the Holy Synod. The High Lords accepted and Thor became the 292nd Ecclesiarch.
He died peacefully six months after this final return, his body exhausted beyond the ability of rejuvenat tech to repair, and was swiftly canonized as a Saint by the Ecclesiarchy.
Sebastian Thor was a major instigator in a mass-unfucking of many of the things that made the 36M Imperium a complete shithole, rather than the merely mostly-complete shithole it is now, and his life meant quite a bit to millions of untold millions of Imperial citizens. For one thing, he more or less reminded numerous Space Marine chapters and the Mechanicus who they were supposed to be fighting for (prior, both factions had basically thrown up their hands and decided to ignore Vandire and the wider imperium's issues as much as possible), and inspired millions more to reaffirm their faith in the Emperor. His work led to the Thorians, a subsection of the Inquisition, effectively being "reborn" after centuries of obscurity. It also led to a death cult that did some suicide bombings and then died out due to all of the suicide bombings, because sometimes preaching about sacrifice without being super specific about what you're sacrificing means some folks will get a very bad idea in their heads.
After his death, his bones and head and such became extremely valuable relics; the Sisters of Battle use his bones as items of worship. A joke among the Inquisition is that if you were to gather all these bones, you'd discover Thor was actually a three-headed, five-armed freak of nature. His head in particular commonly finds its way into the hands of whatever famous plunderer the GW writers want to push is super awesome this year -- most recently Trazyn is the one claiming to have it in his collection, though to the Overlord's credit he claims to have said head in a jar as opposed to a mere skull. Guess we'll have to wait for the Reign of Blood Black Library event to find out.