I guess people have not heard the old joke: When you die, it doesn't matter whether you go to heaven or the other place, you'll end up changing planes in Atlanta!
I've flown through ATL more than I care to remember, sometimes with barely 30 mins. to make a connection. Never had a problem; never missed a flight, even when I had to go from one end of the terminal to another (different) terminal.
40 minutes is close, but possible to do. Probably won't have time for a potty break and if you have to change terminals, you'll need to walk fast.