What is the narrator's explanation of how the devil is really useful and helpful in The Crucible?
When the narrator introduces Hale, he discusses the spiritual and historical significance of the devil. He acknowledges that, prior to Christianity, the gods of the underworld were basically helpful or necessary to humanity's conceptions of higher beings. That is, the gods have always been used to illustrate and justify the right and wrong ways of humanity.
Then he goes on to describe how the Devil has been useful. He says that even in a modern society where the opposites God/Devil are not explicitly mentioned when we talk about the influence of good and evil, we still use the structure of the two opposites: Good/Evil. The narrator says that the church, or church state, has used and continues to use the Devil as a means of keeping people in line. In other words, he is used to scare them or encourage them to follow church rules and doctrine. Different branches of Christianity used the Devil to challenge other opposing branches. Martin Luther was accused of having contact with the devil by the Catholic Church. In turn, he accused them. This was a dispute of Christian ideologies with Martin Luther representing Protestantism as opposed to Catholic beliefs.
The narrator adds that during the time when Communism was feared in America, those associated with Communism were associated with the devil. The color red just happened to have ties to communist symbols as well as symbolism of the devil. So, the devil has also been used for political reasons. In this case, the devil and evil in general were associated with communism. This was the government's way of putting an even more dramatic negative spin on communist sympathizers. When the American government went after communist sympathizers, it was called a "witch hunt." This is a direct reference to the witch trials that The Crucible describes. In fact, Miller is making an allegorical connection between the witch trials and the witch hunt for communists in America in the 1950s. Each side, the Capitalists and Communists or the Americans and the Russians, believed their opponents to have an evil association. The narrator's point is that many groups, religious or political, have used the devil to denigrate their opponents and to keep their own people in line. The authorities in Salem do the exact same thing.