The most high-profile case I think Alex could be referring to here is the Sutherland Springs church shooting back in 2017, but that wasn't stopped by an armed deacon or somebody in the congregation being armed. Stephen Williford was the guy who shot the shooter there, and he was at home when the shooting started, and he only responded after his daughter told him that she'd heard gunshots and he came to the church. He wasn't an armed preacher.