So @abemusedobsessive made this post with their own hc's about Thorin (which I highly recommend checking out), and it inspired me to post my own hc's! Specifically about how Thorin feels about religion.
(For the record, I love the transmasc Thorin hc and will be using it here, and some of my hc's tie into personal experience as a transmasc living in the deeply religious south lol. Anything with Eru is as far as that draw goes tho)
So, I do love their hc's of how Thorin views Mahal, but mine differs a bit. To me, I think Thorin did start out with some anger, maybe even resentment towards his maker. All he was taught was Mahal. He didn't really know much about Eru beyond the fact that he put life in the dwarves that Mahal created; thus, he believed Mahal was in control of the world. Thorin struggled to wrap his head around why such a kind maker would allow such suffering. The deaths of so many friends and family members to Smaug and the following pain that ensued (Grandfather, father, brother, war, etc.)
It only hurt him more when he began to realize that he was... well, a 'he'. Thorin cursed Mahal's name for not only making him witness the suffering of his people (and suffer alongside them) all the while in a body that wasn't right.
As he grew older, he learned more about Eru. How he allowed Morgoth's corrupt harmony to poison the song that made the world, which snowballed into everything bad in his life. Morgoth got Sauron on his side and made him his first lieutenant. Together, they made dragons and orcs and foul things. Not to mention it was Sauron who killed Thorin's father. Not to mention ALSO that Eru had the full ability to stop their pain and did nothing. In a sense, it felt like Eru refused to claim the dwarves as his creations.
Beyond just that, Thorin smithed in the towns of men. In there, he heard all sorts of things about how he had transitioned ftm. He was never shy about it. The dwarves didn't care; in fact, they helped him through every step of the way. But the men? They told him things such as:
"Eru doesn't make mistakes. You're dillusional!"
"A girl you were born, a girl you shall be! Eru would be ashamed!"
"Eru still loves you, even on the wrong path. It's not to late to come back to him, y'know."
And all sorts of other things. Coupling that with the aforementioned blaming, and you have a dwarf who, very loudly, will say, "Eru is not my God." With that thought in hand he starts to turn back towards his proper maker.
Thorin's relationship with Mahal is still very complicated. Part of him wants to trust him as a child to a loving father. To forgive Mahal and, in turn, ask Mahal's forgiveness for blaming all these things on him for so long. The other part of him wonders why Mahal didn't try harder. Why didn't he beg Eru to help the dwarves? Why didn't he do anything when Sauron was being turned? Sauron was once his apprentice (or something along those lines, I forget the term), shouldn't he have seen the signs? Why, oh why, did he not do more to protect the children he claimed to love?
In the end, Thorin doesn't know what to believe. He will proudly proclaim that Mahal is his god and maker to any man who either asks or tries to force Eru, and even to some dwarves. It's better to just keep that turmoil under wraps, as he's sure it would make people very upset to hear their king's distrust of the god who gave him power. But deep down, he is angry at Mahal and even more so at Eru.
