There are a lot of Christians here. I would say a big majority. You just need to connect with them meaningfully on an individual basis. Do the legwork. People tend to form very lose connections here. They follow those with like minded beliefs. If you are open and honest about your intentions to bring Christians together. You do not need a website, marketing expenses, or anything like that.
Nostr users seem to have an allergic reaction to these things.
Treat Nostr like you would as if you asked the priest at the end of Mass to make an announcement to everyone. Hey we are having a get together at this place at this time to talk more about our faith. Join us for a conversation and some good times if you are interested. No pressure whatsoever.
That is really all it would take. Maybe a hashtag at the most so people can readily discover when and where the next meeting takes place.