

The proverbial expression the mills of God grind slowly signifies slow but certain divine retribution.
Plutarch (1st century CE) alludes to the metaphor as a then-current adage in his Moralia (De sera numinis vindicta 'On the Delay of Divine Vengeance'):
Plutarch no doubt here makes reference to a hexameter by an unknown poet, cited by sceptic philosopher Sextus Empiricus (2nd century) in his Adversus Grammaticos as a popular adage:
This expression was invoked by Celsus in his (lost) True Discourse where, defending the concept of ancestral fault, "a priest of Apollo or of Zeus" says:
The Sibylline Oracles (c. 175) have Sed mola postremo pinset divina farinam ("but the divine mill will at last grind the flour").
The proverb was in frequent use in the Protestant Reformation, often as the Latin Sero molunt deorum molae 'slowly grind the mills of the gods' due to Erasmus of Rotterdam (Adagia, 1500), but also in German.
An Italian variation of 1666 reads
The expression was anthologised in English by George Herbert in his collection of proverbs Jacula Prudentum (1652) as "God's mill grinds slow but sure" (no. 743). The German epigrammatist Friedrich von Logau, in his Sinngedichte (c. 1654), composed an extended variant under the title "Göttliche Rache" (divine retribution),
translated by Henry Wadsworth Longfellow ("Retribution", Poetic Aphorisms, 1846):
Arthur Conan Doyle quotes the proverb in A Study in Scarlet:
The proverb is quoted by characters in Agatha Christie's Hercule Poirot's Christmas and A Pocket Full of Rye in reference to the deaths of the victims. It is also quoted in W. Somerset Maugham's The Moon and Sixpence where it is used, somewhat piously, by a family member to imply a certain justice in the demise of the central character Charles Strickland,
When I had narrated his lamentable death I ceased. For a minute or two we were all silent. Then Robert Strickland struck a match and lit a cigarette. "The mills of God grind slowly, but they grind exceeding small," he said, somewhat impressively.
— W. Somerset Maugham, "The Moon and Sixpence" (1919)
During the Second World War, both Winston Churchill and Franklin Roosevelt quoted Longfellow when promising retribution for the extermination of the Jews.
The expression has become a phrasal template, with various words being substituted for "God":
The most common version today mentions "justice", in the legal proverb "The wheels of justice turn slowly, but grind exceedingly fine" (1920).