Faustus is the 1st non-religious book to discuss the Devil. It first appeared in the 1580s. Was it truthful? Yes, partially, Faust was a real man who did live in Germany in the 15th Century with town records to verify his existence. While some town records praise him as a respectable astrologer and medical doctor, other documents denounce him as a necromancer and sodomite in league with the Devil. These accounts gave birth to the literary Faust which has been popular ever since. The idea of making a deal with the Devil fascinates people because it is beyond the ordinary, it could not happen to them, so their imagination picks up this idea and runs.
Most people are generally moral. They live by rules to guide their behavior such as societal laws and religious beliefs like the Ten Commandments. However, this does not mean that they do not think about temptation. This comes around to Lucifer, the ultimate tempter, his name meaning bringer of light in Latin, but what light is there to bring? Before the fall of the angels, this referred to the light of God, but now it means the fires of Hell. People love to imagine the Devil and making a deal with him and how scary that would be, like the thrill of riding a hair-raising roller coaster. Through these kinds of books and characters, they could live a life disobeying the rules and with special powers and there are no punishments because they are merely reading a book and this is just imagination. By doing this, they can keep their moral guidelines.
Stories about making a deal with the Devil are interesting to people because they allow them to use their imagination. People love to live vicariously, especially through reading stories. What better story is there than one with the Devil himself?