Why are religious delusions so common with mental breakdowns?

Sorry to start my response with some clarifications, but your question -- although totally valid! -- is a little difficult to answer due to some potentially loosely defined terms.

First off: what is a mental breakdown? There's a world of difference between breaking down from learning that you have six months to live, to having a manic episode and waking up naked on the roof of a city bus. I'm going to assume that you're thinking of people suffering from schizophrenic hallucinations.

Second: are religious delusions common in schizophrenic hallucinations? Apologies for the pedantry, but as a certified armchair scientist, I like to make sure that the questions I'm researching don't assume an answer. Otherwise, it'd be like walking into a news interview with bias: I'm not going to get useful answers out of someone if I ask slanted questions; I'll get the most information out of the conversation if I stick to descriptive questions.

So the above is to say, I apologize for not answering your question as worded, but I'm instead answering a different one based on what I assume you're asking about 😅: what types of hallucinations are common for people suffering from schizophrenia?

I found a couple decent looking resources online; first, a review paper: The Cross-Cultural Differences in Symptoms of Schizophrenia: A Systematic Review. As is tradition, I skimmed the abstract and results to find a nice looking pull quote (citations have been removed for brevity, so please look at the article for context!):

The studies examined in terms of the content of hallucinations seen in schizophrenia show that the content of hallucinations may vary depending on the culture and region. The contents of hallucinations were mostly critical, intimidating, and imperative voices of loved ones, friends, and acquaintances in the United Kingdom and America). In contrast, religious and cultural themes along with family members and hostile voices were found more prevalent in Africa, Asia, and the Middle East, suggesting that these parameters influence the content of hallucination. When comparing non-western countries to western countries, cultural themes are the most prominent. For instance, Ghana has more religious themes, whereas India has more playfulness and hostile spiritual interpretations. On the contrary, America has more typical diagnostic criteria interpretation.

So it doesn't seem like religious themes are universal, and trends change between cultures. That difference in the content of hallucinations also seems to affect whether they're interpreted as positive or negative: sometimes "the voices" aren't destructive, and have even been interpreted as being playful.

(source Hallucinatory 'voices' shaped by local culture, Stanford anthropologist says)

Among the Indians in Chennai, more than half (11) heard voices of kin or family members commanding them to do tasks. “They talk as if elder people advising younger people,” one subject said. That contrasts to the Americans, only two of whom heard family members. Also, the Indians heard fewer threatening voices than the Americans – several heard the voices as playful, as manifesting spirits or magic, and even as entertaining. Finally, not as many of them described the voices in terms of a medical or psychiatric problem, as all of the Americans did.

Note that that study isn't terribly large -- an interview of a couple dozen people should hardly be used to characterize an entire illness -- but imo it's an interesting perspective nonetheless.

Meta side note: if you could, next time that this is a discussion piece about a topic adjacent to the article linked, could you please link it in your comment instead of in the thread? I'm sure it's not terribly important to anyone else, but I'd appreciate it if links described what they're linking to, since I clicked the CBC article expecting to see a psychology think piece, not another article about car violence. Ever since the Lapu Lapu attack, I've been trying to limit the amount of descriptions of vehicle attacks I run into online.

No worries if that's too much to ask; just figured I'd say so in case it was easy :)