I can't speak for most church goers, because tipping is (and always has been) a case-by-case basis. I tend to believe that if you can't afford the tip, then don't go sit down at a restaurant. Most people who don't tip well (regardless of religious affiliation) are seniors.
I tip a minimum of 15%, regardless of good or bad service. I understand people have their good days and bad days. I can honestly say that I've not ever had a server treat me like garbage, either.
The only time I ever didn't tip is when I went out to eat with my wife and kids, business was slow, and our server didn't show up for 20 minutes. From there, our drink order was taken, we were given our drinks and we sat there for another 20 minutes before we were able to order. After our order, it took over 30 minutes to get our food. People that came in after us had their food, got up and left even before we began. To make it worse, there was no apology from either management or the server. We have yet to go back to that restaurant.