Which would be a suitable alternative for the common idiom "The devil is in the details", without the use of the word "devil"?
No detail is too small.
It's in the details.
Alternative answers are welcome too!
The details of a matter are its most problematic aspect.
The idiom the devil is in the details means that mistakes are usually made in the small details of a project. Usually it is a caution to pay attention to avoid failure.
An older, and slightly more common, phrase God is in the detail means that attention paid to small things has big rewards, or that details are important.
The devil version of the idiom is a variation on the God phrase, though the exact origin of both is uncertain.
Getting into the nitty-gritty
would convey what you mean.
God is in the details
is a common alternative.