Wikipedia seems to think that the idiom “The Devil is in the detail” is derived from an earlier phrase, “God is in the detail”, indicating that whatever we do should be done thoroughly. My personal understanding of this phrase is much closer to that of the Internet site www.UsingEnglish.com - small things that are overlooked can cause serious problems later on.
Bottom line – when we overlook details, that act can come back to haunt us. Generally speaking, it is not easy to rectify the problems that we caused by overlooking those details. This is where the Devil comes in. This gentleman is very connected to the material world. He is aggressive, competitive, lives somewhat in the land of illusion, and acts as a shadow for our psyche. When this shadow is active, we tend to place self-imposed boundaries around ourselves, and may not be able to let go of existing structures.