At some point in the past year I got interested in looking at former churches on real estate sites.
The Onion recently weighed in on these church home conversions: https://theonion.com/converted-church-for-yuppie-douchebags/

Most sites are going to the same data provider, so it isn't super-relevant if you use Zillow or another site. The main thing is that keyword 'church' does not filter and 'former church' brings up what you want.

Is it a thing?

People generally are looking for homes with normal numbers of beds and bathrooms. It's not that they're opposed to other options, just they did not picture their home having a big church-y room and commercial-scale kitchen in it. If you don't care, reduced demand = reduced price for thousands of square feet.

If I had a million dollars saved up, I could see investing in something like this to make some kind of open source / AI alignment / art retreat, permaculture garden, unique TikTok. But in practice I've been spending minimal time on community events and home renovation, and do not know anything about the insurance and financing, so not realistic.

Understand the basic offering of the listing

Is it

a church which is already subdivided into normal condos
a building zoned non-residential or with no obvious sleeping and bathing facilities - if there are no indoor photos, NOPE
a visibly church-y church that you could live in

Investment

If you see a sweet house or apartment for sale, it could be because someone got a better job or is moving closer to family. Churches are not like that.
Churches are built by non-profits, sometimes with property tax exemptions. If a church is for sale, that was not financially tenable. It will be considerably more expensive for you as an individual. Maybe some churches go on the market when someone dies or retires or a national organization is doing asset management, but until I see numbers on that, I assume it's a financial decision.

Churches are sometimes converted into cute Airbnbs or wedding venues. I stayed in a church basement Airbnb in Wisconsin, and the upstairs had pilates or something. But if the church has a valuable commercial side gig, why is it for sale?

A good bet would be a church which is clearly lived-in, or a church which was just redone by house flippers, since you have a good handle on why it's on the market. But if it looks like it only needs repairs or a residential conversion, consider you may be falling into the same trap as the previous owner.

What to search for

Visible problems with roof, water damage on wall or floor
Boarded window or door
References to the building having historic district / preservation rules
Visibly unfinished or gutted rooms (basement OK)
High roof or bell tower that how the F are people going to get up there, especially if there is space up there where an animal could get in.
Person living inside, but when the photographer came all of their stuff was in a nest or pile.

Location

Address is pinned to a real place on the map (not "State Road 67")
StreetView exists there, and the church looks OK currently.
Check church and surrounding buildings for boarded windows or doors. If there are no other buildings around, could be a plus or a minus?
(Repeat above using the history tools in StreetView)
Are there Google Maps reviews from when the church was open? (rare)
Is there a grocery store walking or biking distance away? Is there just the one Dollar General?
How far is it from an airport or functioning train station?
If it's a big church available for cheap in a city, look up nearby house prices. People are trying to get out of these neighborhoods.

Additional Notes

You can search for former banks, too, but churches are getting posted almost every day.
The kitchen photo is usually a real good indicator of when and how much it's been modified for residential.
In any listing where the church and a separate living space are being sold together as a complex for under a million dollars, it's a ton of square footage, and probably way harder to figure out function and pay for heating.
Recently I saw this convent being split from a larger church complex, but it doesn't have its own driveway or parking, so it wasn't clear to me how that would work. Also if you are buying from the neighboring church, I don't know if they can have rules about who's buying it?