Many non-Christians do not believe or feel Christianity is under attack but for most Christians they don't have to have an event like that in Texas to understand that Christians, if not directly (as in Texas) but indirectly via our values and beliefs are, and have been, under attack for many years now.
When my wife ask me who I thought would do such a thing I told her I felt it was a white man and that I believe, when it all comes out, that he will either be atheist or a Satanist. In this shooter's case he was a militant atheist. Now I don't make the same mistake or error some make by associating this individual with all atheist for that is not right or valid but I do believe that there are more just like him who easily could do something similar. It's sad to say but today it has become necessary for many, if not all, Churches to take precautions and have some measure of security at their services.
I do know that some of our local congregations have volunteers that specifically set outside the building or service just to keep an eye on the parking lots and I fully believe those people are armed or at least I hope they are. Thankfully there was an armed citizen that was able to get in a good shot and bring an end to this killing. I have no doubt, although it has not been stated, that the citizen must have got in an eventual kill shot himself that penetrated the body armor or struck the killer such that caused him to drop his weapon. Those wishing for gun control would easily have taken the gun away from the law abiding citizen and the killer would have continued to kill not caring about laws. There is a law against murder (murder is banned) but yet murders happen every day. Drunk driving is banned/illegal yet many more innocent people are killed each year by drunk driving yet you don't have calls to ban and remove alcohol from the shelves of stores. Neither will banning firearms stop this kind of killing.
The reason for it was the evil hatred in this one individual's heart and mind and his choice of the Church was, I believe, his hatred for Christians and their belief along with this atheism. Society is breeding type of individual with its teaching that life is of no special value, not created, but just an occurrence no more special than a roach scurrying across the floor. Hatred is what caused this event to happen and it was easy for an atheist to target Christians but I still believe Christianity is under attack on a much wider scale and will continue to be, from secular Professors and Colleges to Politicians and many Judges to some atheist, Satanist and also from Muslims who specifically target Jews and Christians in many places around the World. I also do not believe it will get better but far worse in the future days/years.