Why Michelangelo should be considered as religious?
First of all, Michelangelo was not the man on the street. He was under protection of a tyrant who admired profane life and encouraged-inspired everyone around him to understand a life and knowledge out of heavy middle ages religious version of life.
We know that Michelangelo was a great fan of Savanarola that he was very impressed by him when he first listened to him preaching when he was 19. Although he was gone under Medici patronage younger. He was born in 1475, he went to Medici in 1489. He was also considerably ignorant compared to other apprentices around him, he wasn't interested in classic education when his father forced him. He got an education it later.
I always think that his family, specifically his father played a lot of role for him not to be able to break through from some sort of stuck between opposites status. Between religiously money greedy, vulgar father wanted to go upper class and a more educated, art loving mother; his humble worker of god in dust nature with aristocratic, glorious life he had to survive. He was not an aristocrat in nature, couldn't act politically when needed and actually his life is pretty much sad behind that gorly of a genius thing. The nature of classical sculpting could have had a lot to do with this.
In those times, probably you know that there is no such thing as being an artist, but just successful craftsmans of certain guilds working for aristocrats, rulers and cardinals and popes. Well also generally in that culture occupations are defined as 'dirty' and 'clean', the best, higher or lower. Sculpting in this sense always caused conflicts among artists and their patronage through time. Considered lowest and dirtiest of arts, because it is technically about chipping stones in dust, playing with mud, while drawing and painting is more 'intelligent' and a 'clean' job. (There is a reason why Leonardo didn't do traditional sculpting, but cast bronze sculptures. Probably even them just because he had to.) Which sounds so funny to us now. It's also much more expensive, takes a lot of time and it comes and goes out of fashion very fast. (There is also a general 'fear', conflicted feelings against 'sculpture' you can observe through history. Never treated as some ordinary art like others.)
It always felt to me like Michelangelo resented this judgement. When you read about him he is always presented as 'painter and architect' as it should be, but it's obvious he disliked painting and as a reaction he saw it as a 'lower' art because it is not 3 dimensional. There was god's creatures, life in stone. He helped them to get out by removing the excess. What is there in painting?
So his prudence or what I expressed with a hyperbolic way, his religiousness is also related about his place among other artists; as a sculptor working with stone (flesh) like god made man. Direct material to work with. Painting is a reflection and very undirect. He worked in life size or bigger. People who commisioned him certain style of works to celebrate a specific culture. While the reason Lorenzo chose him for his ability of producing art works resembling classical beauty -which conflicts christian understanding of aesthetics of that time, it is just about to be born- Michelangelo sculpted those with a simpler understanding of god's truth of man's nature. Yes, later he did got educated in that culture, but he has always been loyal to stone and reflecting god with it other than making progress to some simple style. Because like Dürer, Mcihelangelo didn't need to study neoplatonism to promote that artistic style. He already had it, it was what he saw when look around.
The only reasons Michelangelo accepted to paint Sistine Chapel is that he was responsible for sending money to his father -who basically did nothing but badger him for money- he was ordered by a pope and because the pope promised him the greatest sculpture garden ever made -pope's tomb-. It didn't happen. He also almost got blind painting that stupid ceiling because of an accident.
His latest pietas are designed to show himself as the 'altar' holding christ after taken down from the crucifix, while his most famous pieta of young years is presenting a christ in the altar like a sacrifice to God -held by the Virgin, who he depicted as beautiful and young woman because he believed women didn't get old or ugly when they remained virgins.
That's a big gap. Most of the artists saw themselves as a medium between god and people, carrying, visualising his word. He saw himself as a part of that divinity, that ability to 'create'. What he did was sacred to him far more than others. He is a different sort of fanatic.
E: PS: Was Michelangelo a 'jerk'? He certainly was. He got his nose broken, because he heavily insulted a work of a close friend (Caracci? don't remember) in the workshop. It was blasphemy to him not to be able to depict 'man' as god created him. He is also known for his strong grumpiness and stubbornness.