@literally45-- That's a great question. There are religious themed oratorios that are not about Christianity. As far as I know, so far there have been oratorios written for Buddhism and Hinduism.
"The Light of Asia" is about Buddha for example. And Ilaiyaraaja, whose works are very popular in South India, wrote an oratorio inspired from Hindu hymns. I know about this because I've seen many films where the music was written by Ilaiyaraaja.
I'm sure that other non-Christian oratorios will continue to emerge in the next decades. It's actually very interesting how oratorios continue to be made, performed and listened to. If I'm not wrong, they had lost their popularity briefly in the 1800s. But they appear to be regaining popularity again now.