So I was about to ask a question on christianity.stackexchange.com and I got an error of "you're about to get question banned."
Now, I am curious -- how am I close to being question banned despite having a positive reputation (i.e. 225). ?
This isn't a question we can answer.
The algorithm for establishing question bans is secret, so frankly we don't know, and couldn't tell you if we did.
That said, looking at your question history, even though you have managed a positive reputation score, most of your questions are downvoted and/or closed.
I would advise you to edit and improve your existing questions prior to asking additional ones.
Or if you do feel led to ask additional questions, spend extra time making sure they conform to this community's guidelines laid out in the help center.
This happened to me recently on a different Stack Exchange site. I personally was a bit baffled because I consider myself a positive contributor on that site.
As was already said, the way they decided if you should be question banned is only known by the SE administration. This puts you in a weird spot that pretty much everyone else feels when it happens to them.
When I saw it, I looked exclusively at my ten newest questions. I had one -1, one -3, one +1, and the rest were 0 score. Overall, I was alright, just on a bad run I guess. My next question; which I made certain was well formed, definitively answerable, and on-topic; brought in four upvotes. Now I don't see the message anymore.
I recommend that any future questions you ask are well formed, definitively answerable, and on-topic. See the Types of questions that are within community guidelines. Absolutely do not make your next question any other type except one of the first six. Those first six are proven question types. They've been on-topic for a long time and are usually inherently definitively answerable. All you have to do is form it well (grammar, understandable, concise but detailed, etc.).
Also, just to be sure, run the question by us in chat first. We'll help you if you seek it out.