If the attacks were happening on JW, Mormon, Scientologist, Muslim, and other high control churches I'd agree. But on liberal churches that really let people do anything they want, not too much into their personal lives, don't do shunning, etc, I do not think that is the motivation.
Also many mass shootings happen where people traditionally don't have guns. I don't think any laws for concealed owners to not have one in a church, but bars, schools, etc, are easier targets.