Mills of God

The proverbial expression of the mills of God grinding slowly refers to the notion of slow but certain divine retribution.

Ancient Greek usage
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:
 * "Thus, I do not see what use there is in those mills of the gods said to grind so late as to render punishment hard to be recognized, and to make wickedness fearless."
 * Ὀψὲ θεῶν ἀλέουσι μύλοι, ἀλέουσι δὲ λεπτά.
 * "The millstones of the gods grind late, but they grind fine."

The same expression was invoked by Celsus in his (lost) True Discourse. Defending the concept of ancestral fault, Celsus reportedly quoted "a priest of Apollo or of Zeus":
 * Ὀψὲ, φησι, θεῶν ἀλέουσι μύλοι, κἆϛ παίδων παῖδας τοί κεν μετόπισθε γένωνται.
 * 'The mills of the gods grind slowly', he says, even 'To children's children, and to those who are born after them.'

The Sibylline Oracles (c. 175) have Sed mola postremo pinset divina farinam ("but the divine mill will at last grind the flour").

In 16th and 17th century Europe
The proverb was in frequent use in the Protestant Reformation, often in the Latin translation Sero molunt deorum molae due to Erasmus of Rotterdam (Adagia, 1500), but also in German translation.

The expression was anthologised in English translation by George Herbert in his collection of proverbs entitled Jacula Prudentum (1652), as "God's mill grinds slow but sure" (no. 743). German epigrammatist Friedrich von Logau, in his Sinngedichte (c. 1654), composed an extended variant of the saying under the title "Göttliche Rache" (divine retribution),
 * Gottes Mühlen mahlen langsam, mahlen aber trefflich klein, ob aus Langmut er sich säumet, bringt mit Schärf 'er alles ein.

translated into English by  Henry Wadsworth Longfellow ("Retribution", Poetic Aphorisms, 1846):
 * Though the mills of God grind slowly; Yet they grind exceeding small;
 * Though with patience He stands waiting, With exactness grinds He all.

Modern usage
Arthur Conan Doyle alluded to the proverb in his very first Sherlock Holmes adventure, A Study in Scarlet. The allusion is found in the fourth chapter of the second part, in a scene in which the character John Ferrier is confronted by two of the Mormon characters:
 * . . . Both of them nodded to Ferrier as he entered, and the one in the rocking-chair commenced the conversation.
 * “Maybe you don’t know us,” he said. “This here is the son of Elder Drebber, and I’m Joseph Stangerson, who travelled with you in the desert when the Lord stretched out His hand and gathered you into the true fold.”
 * “As He will all the nations in His own good time,” said the other in a nasal voice; “He grindeth slowly but exceeding small.” John Ferrier bowed coldly. He had guessed who his visitors were. . ..

Arthur Conan Doyle, A Study in Scarlet (1886) (emphasis added).

The proverb was used by Agatha Christie in her novel Hercule Poirot's Christmas, as a person quoted it when they saw the corpse of a man who had lived an evil life. It was also referred to by W. Somerset Maugham in the novel The Moon and Sixpence wherein it is used, somewhat piously, by a family member to imply a certain justice in the demise of the central character Charles Strickland,

"Then I told them what I had learned about Charles Strickland in Tahiti. I thought it unnecessary to say anything of Ata and her boy, but for the rest I was as accurate as I could be. 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

During the Second World War, both Winston Churchill and Franklin Roosevelt quoted Longfellow when promising retribution for the extermination of the Jews.