Before the 1800s most of the descriptions of Os and Cs deal with religious topics. For instance, it is easy to find descriptions of obsessive blasphemous thoughts and compulsive confessing of sins. From the 1800s on, though, few of the Os and Cs found in published descriptions have religious themes. Is it actually true that religious Os and Cs have become less common over time?
Probably. Researchers who compare OCD in different cultures today have found that highly religious societies tend to have more religious OCD symptoms than are found in more secular societies.1 Maybe the same principle also applies to changes over time: if a society gradually becomes less and less religious, perhaps there are fewer Os and Cs with religious themes. In other words, maybe OCD symptoms today are not entirely the same as they were historically. It makes sense that society and culture would have some influence on Os and Cs. Maybe, as the world changes, it causes an important ripple effect in the forms that Os and Cs take.
1See L.F. Fontenelle, et al., Trans-cultural aspects of obsessive-compulsive disorder: a description of a Brazilian sample and a systematic review of international clinical studies, J. Psychiatr. Res., Jul.-Aug. 2004, 38(4):403-11.
*For further discussion of this general topic, see Strayer, M., What was it like to suffer from obsessional disorder in the United States, 1840–1950? Journal of Medical Biography, 2018, 26:125-131, 128.