Two rules:
Rule One: When a shooter appears, everyone in church, if they cannot immediately escape, hits the deck. The shooter will remain standing, but now he’s the obvious target for those tasked with security; he is “acquired” and “isolated” with no one to hide behind. Those tasked with neutralizing the threat will know what to do depending on the policies of the church, either rushing him until he is immobilized (with possibly lots of people needlessly being killed in this scenario) or by – on their own authority in this diocese – using the proper tools to deal with him (with possibly much fewer people being gravely wounded, maimed or killed).
Rule Two: When a shooter appears, everyone in church, if they cannot immediately escape, hits the deck.
This post assumes that there is a someone or are “someones” who are tasked with security. I suspect this is not the case in most places. Some of the churches with which I am familiar do not offer an escape route, so I suspect it would would have to be more a matter of enforcing deliberate chaos to keep the gunman off balance so at least some can escape before they are methodically killed. The only weapons at hand in the nave would likely be hymnals. From what I have seen of a number of hymnals, their being used as manual projectile weapons against an intruder is probably a better use than singing what is printed inside anyway….