Church security starts outside. Counting on armed parishioners or security volunteers inside the structure is--literally--an act of closing the barn door after the horse has been stolen. If guns inside the church/temple/mosque are of any use, then you are already having a Very Bad Time.

Clear inside, secure all entrances, establish a perimeter, and then control access. Lay eyes on everyone coming in, keep close watch on newcomers. Keep people moving to minimize the crowd on the way in. If you're going to have a problem, have it outside.