This is a 2 different quotes in 1. The devil is in the details, originated as God is in the details. The usage of the first later overtook the usage of the original, and i will explain why later on.
God is in the details.
The origin, is either French or German.
“Le bon Dieu est dans le détail” (French for “the good God is in the detail”) is generally attributed to Gustave Flaubert (1821–1880).
“Gott steckt im Detail” appeared in german print in 1934. The expression also have been a favorite of German art historian Aby Warburg (1866–1929).
God is in the details : Means that perfection require to pay attention to the small details. It can also mean, that doing a good job, a God’s job, require that you take care of the details, and try to reach perfection.
The devil is in the details
The devil is in the details: Is an attitude shift from the original quote, that’s all. It is a change of attitude towards the task, job or agreement, instead of seeing it in a positive light, now we look at it in a negative light. Now the meaning shifts into something entirely else. Now, it means a small detail can ruin the task/job/agreement etc … And the offer should be looked over very carefully.
Both quotes call in general, to pay close attention to details.
Another interesting quote about focusing on details is:
The man in charge must concern himself with details. If he does not consider them important, neither will his subordinates. – Hyman Rickover