The article at the link includes a graphic showing that only 15.16% of churches have security measures in place as a safeguard against terror-related attacks.
Some thoughts, not necessarily in any kind of order:
Security in churches is far more likely to involve dealing with ex-spouses and the mildly deranged, than actual violence.
Firearms trainer Jon Hodoway has some reflections on church security, I think here.
Most places, by law, churches are victim disarmament zones.
And, as a historical example that can be learned from, the St. James Massacre, from apartheit era South Africa. An ANC terror squad started to shoot up a church, and were driven off my a man with a 5 shot .38 snubby. Charl van Wyk's story: Shooting Back.
Thanks for the great links.