No, it’s not really fan art. It just happened one of the largest art patrons was the church. They wanted art to “educate” (i.e. evangelize) a largely illiterate

No, it’s not really fan art. It just happened one of the largest art patrons was the church. They wanted art to “educate” (i.e. evangelize) a largely illiterate population. And there was no internet back then, so every religious site would need art illustrating the same themes.