I believe thr phrase is actually 'the devil is in the detail' which infers that any situation that might have complications will reveal those complications when everyone gets down to examining the details. That's when the problems and issies - the devil - are revealed.
I am sure Quizzmonster will be along with a typically erudite and complete explanation, if you can manage with mine until he gets here. Thanks.
How kind of you, Andy. However, I can but agree with both you andJake in the sense that the 'devil' version is now far more frequently heard than the 'God' one. It does not appear to be recorded who actually was first to use the former.