If they ask you to leave and you refuse, it’s trespassing. Doesn’t matter if they are open, etc, etc.
Now, you could go back and sue them and probably win if they had no reason to ask you to leave, but you have no right to resist arrest simply because you think you should be allowed to stay somewhere when they are asking you to leave.
If the event was open to all, and she was doing what was reported, which was being silent in prayer, and not blocking, disrupting, or destroying anything, and, apparently, not there when the facility was closed, by what authority was she to be removed? Would they have removed someone who was just sitting there, doing nothing at all?