Interesting scenario. I work with a lot of churches teaching them how to plan ahead to avoid ever having an active shooter event and how to deal with them in the case they do.
The most obvious and simplest thing to do is limit access. Secure the doors so that only the main entrance of the worship hall is accessible during services. It’s fine to have locks that allow for easy/instant access from the inside in case of emergency but that don’t allow for easy access from outside.
Position at least two ushers at the entrance and have them greet people as they enter. It usually doesn’t take long to figure out if someone is mentally unbalanced or has ill intent when you make eye contact, greet them with a smile and reach out to shake their hand.
If you’re going to have a problem, the vestibule is the place you want it happening.
Having at least two additional people you can rely on close to the entrance aside from the ushers is also a very good idea.
Four to one is very good odds if the four are of the right mindset to prevent bad things from happening.
If someone seems questionable but obviously not a problem, someone should escort them to their seat and join them. You don’t have to be overly obvious about it but vigilant.