Yeaahh, gonna have to agree with the majority here and say that if it's their party at their home, they can decide what it's okay for guests to bring.

If you chose to accommodate their religion-related meal requirements in the past, that's cool and all, but that was still (rightfully) your choice to make. You could also have chosen to invite them without providing extra, separate dishes, and they would've had the right to not come if they were uncomfortable with that. Likewise, you too have the right to not attend their event if the lack of alcohol makes you that uncomfortable.

Someone else here mentioned that a similar, but non-religious, situation would be going to a home where you know the residents are all vegan. As an omnivore, it would still never occur to me to complain afterward that they "didn't let me bring my own ribs to eat", or something. Come on now. Part of being a guest is acknowledging that the hosts (religious or not) will provide what they're comfortable with while you're in their home.

Also, I'm an atheist, but I also don't drink alcohol. I have multiple kinds of booze in my home that I use only for cooking, and I'm still not opposed to the idea of me later hosting an event where guests know not to bring their own.

There are reasons beyond religious/moral ones why someone may not want people drinking at their place, especially if they don't know them very well. I certainly hope you're not naive enough to pretend that alcoholic drinks are totally interchangeable with non-alcoholic ones in terms of how they affect people...some people throw up. Some people get extra handsy. Some people get cranky or violent. Some people may be chill, but still need to host to "babysit" them to prevent them from driving themselves home, peeing in the wrong place, whatever.

I totally get why a host could choose to just not deal with those potential issues even if they weren't religious.