The Holy Father offers some important tips:
161. Hence, we should not think of the devil as a myth, a representation, a symbol, a figure of speech or an idea.*
See the important note below for Pope Paul VI’s assessment.
This mistake would lead us to let down our guard, to grow careless and end up more vulnerable. The devil does not need to possess us.
The notion of possession is rooted in popular culture. That might be worth some analysis. But the notion of maintaining a vigilance with regard to evil–this is especially important for those who prefer to clothe themselves in virtue. If we have dipped too deeply into the pool of pelagianism, thinking well of our own virtue, watch out. That may be the perfect moment to be ensnared in sin.
He poisons us with the venom of hatred, desolation, envy and vice. When we let down our guard, he takes advantage of it to destroy our lives, our families and our communities. “Like a roaring lion, he prowls around, looking for someone to devour” (1 Pet 5:8).
This note would have been written after the appearance of William Friedkin’s novel, but before the film adaptation:
* Cf. PAUL VI, Catechesis, General Audience of 15 November 1972: Insegnamenti X (1972), pp. 1168-1170: “One of our greatest needs is defense against that evil which we call the devil… Evil is not simply a deficiency, it is an efficiency, a living spiritual being, perverted and perverting. A terrible reality, mysterious and frightful. They no longer remain within the framework of biblical and ecclesiastical teaching who refuse to recognize its existence, or who make of it an independent principle that does not have, like every creature, its origin in God, or explain it as a pseudo-reality, a conceptual and imaginative personification of the hidden causes of our misfortunes”.
Remember that you can check the full document Gaudete et Exsultate on the Vatican website.