One thing about the ASoIAF books that had annoyed me was how as the series went on and GRRM filled in the cultural worldbuilding stuff, how much the religions - with the exception of the Old Gods, who are reskinned Germanic paganism - were riffs on Christianity.
The Faith of the Seven started off as kind of novel polytheism, but as the fluff grew it accumulated a heaven and hells and holy scripture and stained glass and a Pope High Stepton.
The religion of the Drowned God is Protestantism, with a focus on baptism (even arguments over the validity of infant baptism) and unlettered charismatic preachers inspired by life-changing rebirth experiences.
And the Red God gets the resurrection stuff, and the ritual sacrifice of a chosen son, and the angry/jealous “no god before me” stuff (there’s a bit of Zoroastrianism in there too, with the fire worship and the R’hllor/Great Other bit mirroring Spenta Mainyu/Angra Mainyu).
But I’ve actually come around on that a bit - one of the triggers was Rod Dreher wondering whether there’s actually anything distinct about cultural (as distinct from religious) Catholicism in America and asking whether there were any culturally Catholic writers, and and someone nominated GRRM.
(For the record I was raised nominally Irish Catholicish by my former altar boy father, taken to weekly masses, took communion but was asked not to come back to CCD before confirmation [as I had been asked not to come back to that parish’s school after kindergarden, as I had been kicked out of three preschools, as I probably would have been asked not to come back to public school if my father wasn’t the district’s lawyer], probably to the relief of both sides and to the bemusement of my mother, nominally German Protestant from a midcentury mainline tradition that didn’t so much believe in God as in being better than those filthy Papists)
Because I guess the thing about nonreligious Catholic identity is you can reject the idea that this mythology is true, that these divinities exist, that these doctrines are correct, that the notions of sin and salvation are important. But you’re faced with the fact that the Church itself was a real thing, something that existed and insinuated itself into every single aspect of life in Western culture, from thousands of years ago to, in pockets, the modern day, and if that can’t be attributed to inherent truth, than it demands otherwise accounting for.
And the whole schtick of the ASoIAF books is this feudal realism, making the point that so much fantasy confuses the legitimating myths of feudal society - chivalry, nobility, legitimacy by royal descent - for the actual mechanisms by which it operates, which were realpolitik as always. (I mean, I shouldn’t act so superior here, I used to be into the kind of civil libertarian constitutionalism that does the same thing with democratic society.) And as I thought about it, I realized that while GRRM parceled out bits of Christian doctrine amongst the religions, they had basically nothing to do with what things came to pass. But at the same time, he parceled out bits of Christian function which does.
The Faith of the Seven is the rule of Rome, entwined with the status quo system - kings are crowned and knights invested in their name, their preaching legitimating the state of things and shunting dissatisfaction into the promise of an afterlife. The election of the High Septa, nominally in the hands of the curia of the Most Devout, is subject to pressure from secular leadership. But for all that, their role as the repository of legitimacy, nominal in times of peace, can in times of misrule and fractured rule become an alternate power base to challenge secular authority. And however corrupt the institutional church is, it still produces, and as with the Sparrows occasionally falls into the hands of, people who really believe in the populist doctrines. In its ubiquity, it serves as a core around which the little people can organize and enforce their claims against the rule of warriors.
The Drowned God is Protestantism, yes, powered by individual charisma, but this lack of formal structure renders it vulnerable to subversion. Earnest born-again preacher Aeron Damphair, the most faithful, calls a revival meeting/kingsmoot with intention of inspiring a renewed faith, but in the end Euron Greyjoy the heretic wins the men over with his display of treasure, and promises of more to come. The charisma of piety competes on level ground with the charisma of strength and fortune, and loses, humble faith giving way to the gospel of wealth.
The Red God is I think the most interesting case - Christianity as experienced from the receiving end of a missionary effort. Arriving from a distant land, acquiring as patrons dissident nobility with with the promise of the weaponry to claim power in their own right - steel and gunpowder at various stages of Christian expansion, Melisandre’s shadow magic in the books. Particularly intriguing is the implication that there’s more at play than the new converts realize, that awed by the twinned display of power and demand for exclusive fidelity they lack the perspective to realize that the two are not actually related, and that their quest for dominion, if realized, would actually result in becoming a mere satrapy of a greater empire.