The idiom "the devil is in the detail" refers to a catch or mysterious element hidden in the details, meaning that something might seem simple at a first look but will take more time and effort to complete than expected and derives from the earlier phrase "God is in the detail" expressing the idea that whatever one does should be done thoroughly; i.e. details are important.
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Origin
The idiom "God is in the detail" has been attributed to a number of different individuals, most notably to German-born architect Ludwig Mies van der Rohe (1886–1969) by The New York Times in Mies's 1969 obituary; however, it is generally accepted not to have originated with him. The expression also appears to have been a favorite of German art historian Aby Warburg (1866–1929), though Warburg's biographer, E.M. Gombrich, is likewise uncertain if it originated with Warburg. An earlier form "Le bon Dieu est dans le détail" (the good God is in the detail) is generally attributed to Gustave Flaubert (1821–1880). Bartlett's Familiar Quotations lists the saying's author as anonymous. The phrase is attested in 1965; in 1969, it is referred to as an existing proverb.
Variants
The phrase has several variants: (The / A) Devil (is) in the Detail(s). The original expression of "God is in the detail" most likely had the expression ending with "detail" (without an s), colloquial usage often ends the idiom as "details"; detail, without an s, can be used as both a singular and collective noun.
More recently, the expressions "Governing (is) in the Detail(s)" and "(The) Truth (is) in the Detail(s)" have appeared.