The phrase the devil is the details is likely an inversion of the older god is in the details. I find one far more profound than the other, and I want to articulate why.
First, some history. The god version is likely older, though the specific lineage of it is murky:
We know that Mies van der Rohe used it in discussing architecture; Flaubert has been suggested, but nobody can find it in his writings. I think it may come from John Ruskin, because it sounds like him on the subject of workmanship, but we need the specific citation.
But although it is fascinating enough that both phrases exist, what is more extraordinary is how the balance between them has changed over time. The Google trend data shows that the devil has long reigned supreme:
And that the trend is similar both in German:
And in French:
I don't know to what extent idiomatic usage is correlated among these three languages, but I still find the overall trend striking. In three different cultures, there is a clear preference for the devil living in the details.
The devil is in the details means that God is elsewhere: in abstractions, in plans, in some place beyond test or threat. In this view, the devil is in the details because details undermine our ideals and dreams. They vex us by constantly exposing how ignorant we are of the ground under our feet. And since reality is constant detail, it follows that God is not in reality.
God is in the details means that God is everywhere: in writing Python, in doing the dishes, in the whole mess of life. The more we seek detail and understanding, the more richness we find. So where is the devil? Sleeping in our delusions and sky castles. Unknown and untested, he arises when we lose touch with the loam of reality and forget where we stand.
I think both phrases are true to different attitudes to life and craft. But if you've been following along, it's no shock that I prefer the second, which I think is far more conducive to meaningful action in the real world. Then our ideals are in service of what we can do here and now, in the only world we can be sure we have.
As for why there's been a shift, I don't have the intererst or expertise in social critique to say. But my armchair conjecture is that modern society is so disorienting and disempowering that we no longer feel confident about what is real or meaningful, and we no longer have confidence that we can thrive in the reality we have.
And if that conjecture is true, the next step is to understand the world as it is and build a better one, moment by moment and step by step. Let's get to work.