In the Industrial Era, much of the West Coast was secular in nature, with basic Christian tenets merely informing its lifestyle. From Las Vegas to Los Angeles to San Francisco, much of the regions west of the Rockies had, at best, a tenuous relationship with the country's religious leaders. However, once society slid back into medieval era, people were going to slide back into religion no matter what. All it needed was something to fill the vacuum.
The Church of Scientology started out as a fringe cult fronted by eccentric celebrities. This tended to be a boon when society started devolving. Even though performers have little practical use in a back-to-basic civilization, wealthy performers may have a little more clout. Estates of the rich and famous would be lent out to the peons, and celebrities could maybe amass a few loyal bodyguards. Some may have even had enough foresight to stock up on goods and hide in a personal complex until the worst was over, and they could come out to pick up the pieces.
This is where the Church of Scientology stepped in. Often criticized for its bizarre ideas and scare tactics, the fact was, nothing really stood out as insane as somebody had some kind of out-there religious epiphany on every corner. It's just that the CoS was well-equipped with charismatic and already well-known preachers. Many of the huddled masses even found comfort in familiar faces from better times, and a tightly rigid structure. Sure hierarchy was brutal and treated them as little more than slaves, but this was par for the course everywhere in the desert, so it might as well be them. Laws against detergents and psychiatry mean rather little when they're not around anyway. Criticisms on how credible the back story is also bounce of a populace that is mostly illiterate and superstitious anyways.
While things like Xenu, Thetans and Auditing still exist in the Church, a large amount of the canon has slowly been permuted with what's known as "Hollywood folklore". It's filled with heroes like Indiana Jones, goddesses like Marilyn, wizards like Vader, monsters like Kong, and locales like Casablanca. The Mann's theater is a place to canonize their heroes and saints, and rulers are appointed with a gold statue of a man at their coronation. It's said that upon death, the President of the Church will join his ancestors and become a star in the sky. These figures of Californian mythology appear in a lot of art and sculptures, as the CoS is the largest employer of artisans and masons, and often wish to make their places of worship ornate and easy for the peasants to understand. Californian art and architecture also has a recurring motif of rings and arches.
Interestingly, many of the Holidays of the Church match the Non-Denominational ones. Festivals are held around the same times Thanksgiving, Christmas, and the Fourth of July are in the East, however the celebrations are strictly held to the weekend. There is also a major Memorial Day, to commemorate those destroyed by Xenu. (Because there are not a lot severe winters in California, spring festivals do not feature in Scientology very much. More often Scientologists use spring as a time to discard or use up lesser possessions) These festivals are known for producing large theatrical productions which tell stories of many great heroes and events in the culture. November tends to focus on stories for children, December pageants tell their stories with song, and July presents stories of the most heroic and bombastic feats.