When I read the first verse, I actually thought you were joking and this was a parody of the real song. Ouch.

"It's probably not the effect our dear Mr Secular-Humanist Pratchett would have wanted, but this image has had a profound effect on my own faith in a good way."

I don't see why not. To me the whole book basically seemed like an effort to make people see why it's so much better to build their own relationship with the deity of their choice rather than trust other people to know what's really true and what isn't. It pokes fun at religion, but not so much at faith itself. As far as I know, Pratchett is an atheist, but at least in his books he has sympathy for those who believe.