Short explanation: One of the sneakiest lies from the father of lies is that he doesn’t exist.
More complete explanation:
Some self-styled “theologians” claim that Satan is merely a literary anthropomorphism invented by the authors of the scriptures to personify evil, much like we use Uncle Sam to personify the United States. This is not true. Jesus referred to Satan and other devils on numerous occasions as real people, not as literary characters. Someone may object that Jesus didn’t really say that, but had those words put into his mouth by Matthew, Mark, Luke and John. This is not true either. The Church has always taught, as a tenet of faith, that Satan is a real person, that the other devils are real persons, and that Jesus meant exactly what he said.
Scripture, Church Tradition, and the Councils of the Church have all unanimously taught that some angels, who were made good by God, became evil of their own free will, and fell from grace. That they have had, and continue to have, a disastrous influence over humanity, is attested to in many chapters of the current Catechism of the Catholic Church. The official Rite of Exorcism would not exist if devils were only imaginary beings.
Does Satan really exist? Yes. Don’t let him tell you otherwise.