The proverbial expression of the mills of God grinding slowly refers to the notion of slow but certain divine retribution.
Plutarch (1st century AD) 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:
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":
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 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),
translated into English by Henry Wadsworth Longfellow ("Retribution", Poetic Aphorisms, 1846):
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.