The gunman had some serious, serious weaponry, people in church are often vulnerable because to make it really safe, you'd need the ushers to be armed as they are in the back. Most everyone else is you know, in front. The gunman had a semi-automatic, a lot of concealed weapons might be pea shooters compared to that.
There are, in some cities, dozens and dozens of churches and it appears to be actually, hundreds and hundreds in metropolises. So, it will be up to each individual church to decide on security.
Let's dispel some of the myths there.
The gunman entered the front of the church, not the rear. For decent security, you'd need people in the front and back.
Semiautomatics are like internal combustion engines. They come in different sizes. It might be a .22, it might be a .50BMG. The caliber is what determines what damage an individual round will do. In this case he had an AR type 5.56mm (.223) rifle. Standard magazine capacities for that rifle are 30 or 20 rounds. Potent enough, but many concealed carriers carry .45 semiautomatic pistols, another potent and battle proven round. What tips the balance in favor of the shooter is the body armor, forcing any defender to shoot for the legs, a gap in the armor (usually at the sides or shoulder/armpit), or to try for a head shot. The latter two are not easy to hit on a dynamic but nonthreatening target under ordinary circumstances, harder still when the target shoots back.
Even though body armor prevents penetration, without a trauma plate, the impact is still felt, there is still energy transfer. A couple of shooters in the front, engaging at center of mass would have made his day more miserable and impacted his effectiveness, as well as his torso. While those impacts would likely have left only bruises, they would have messed up his control of the situation, and possibly given the defensive shooters time to get that head shot.
When I was a kid, shooting up a house of worship--regardless of which religion--was just considered an especially vile thing to do, real barbarian stuff. But we were a more moral society then, and even the criminal element generally acknowledged that there were some things you just didn't do. Not so any more, apparently.