The war referenced in this passage is presumably World War I (1914–1918), as the novel was published in 1919, the year after World War I ended. The phrase “The mills of God grind slowly, but they grind exceeding small,” which refers to divine punishment for sin, is not originally a Christian proverb—it derives from ancient Greece and originally referred to “gods” rather than “God”—but came into common usage in European countries in the 1500s. The narrator’s comment that the Stricklands probably belief the quotation is actually from the Bible implies that they are hypocrites who don’t know their own religion particularly well. The phrase “the devil could always quote scripture to his purpose,” meanwhile, is a paraphrase of “the devil can cite scripture for his purpose,” line 105 from Act I, Scene 3 of William Shakespeare’s
The Merchant of Venice (c. 1596–1598). It means that an evil person can say good or holy things to appear good. Here, the narrator is implying that the Stricklands are wicked, hypocritical people for suggesting that Strickland got what he deserved in dying of leprosy—and indicates that the narrator ultimately sides with Strickland in his conflict with conventional society against people like Mrs. Strickland and Robert who continue secretly to hate him for his behavior.