There's nothing more jarring than reading a blistering email filled with "I hate, hate, HATE this" and "I don't care why, just FIX IT!!!" and then seeing it signed "In Christ," or "God Bless!"
I'm sure Lewis must have said something clever about this kind of thing in Screwtape Letters or Great Divorce, and not being nearly as clever as Lewis, I won't try.
However, one thing that's been brought home to me lately in other areas is that you never know when you're unknowingly serving as an example of what it's like to be a Christian. (Or whatever other system of beliefs you may be a proclaimed member of.) Which makes seeing rudeness followed by proclamations of religious sentiment doubly troubling.