The above colloquial expression is a variation of "God is in the details”, meaning whatever one does should be done thoroughly; details are important. The variant, “The Devil is in the details”, expresses the idea that there is a catch hidden in the details.
One can easily imagine that the originator of this colloquialism had the theology of Christian Orthodoxy in mind when he or she coined this expression. The existence of a Devil’s Hell as the final abiding place of most of humanity is the “fly in the ointment” of the traditional church’s gospel or “good news”. The story of God, Christ, and redemption always starts out as an upbeat tale of love, mercy, goodness, and light, but sooner or later the tellers get around to the missing details, those about wrath, judgment, the Devil, and the ill fate of the majority of humanity. Under the church’s paradigm the gospel eventually morphs into the worst kind of bad news. It degenerates into a variation on the commercial tactic of “bait and switch” except that the switch takes the unwary “customer” to something that no one would ever want to buy.
Of course, one could conjure up many additional ways in which the expression, “The Devil is in the details”, is an apt description of Orthodoxy. Many aspects of orthodox theology have associated implications and unanswered questions which undermine its credibility. As long as the church can get by with quoting isolated scriptures in support of their doctrine without having to deal honestly and rationally with the details and ramifications, they can maintain control of the dialogue. But as soon as they are forced to address the details of applying their theology to real life situations, they are in a “devil of a fix”. That is the reason why scripture quoting is so popular; it lends an appearance of validity without having to address these unseemly, devilish details. Yes, details are important, much too important to be dismissed by a conveniently quoted verse or some veiled reference to God’s unfathomable ways.