My friend Robert Royal has an interesting, and typically thoughtful, piece up at his blog "The Catholic Thing" in which he first notes the way Pope Francis does not fit into standard ideological categories, a happy thing to note, but also points to the new pope's frequent references to the Devil. I have noticed this also.
Here is a real challenge to modern ways of thinking. The idea that evil is personified is problematic to the modern mind. It is impossible to have lived in the same century as Auschwitz or the same decade as Srebrenica and deny the existence of evil. And, it is obvious that a too-easy personification of evil can lead to gross misunderstandings of a theological sort, a kind of Manichaenism. Against that heretical temptation, we have the sound doctrine of Augustine who held that evil was an absence of the good. But, how can "an absence" permit a personification? This is a question I will leave to greater theological minds. But, I will say this. Up until a few years ago, I was content in the modern idea that the Devil was a metaphor, a way that earlier and more benighted folk came to terms with the existence of evil. Then, one night, a friend attempted suicide and I was acutely aware that I was in the presence of something demonic. There was pride mixed in with the desperation of the act, a "look at what I did" conflation of the two seemingly opposing realities of pride and desperation that only a demon could produce. It was as palpable as it was inexplicable.
Then, a wise theologian friend explained to me something that did not resolve the tension between pride and desperation I had witnessed on that dreadful night, but shed the light of faith on it in a way that had not occurred to me. He explained that the Church requires that a priest who is going to conduct an exorcism must be a very holy priest, because in the struggle with the demon, the Devil will tempt the priest to despair by holding up his own sins to his conscience. Only a holy priest, that is, someone who holds fast to the sure hope in the Resurrection, only such a priest can resist the invitation to despair with sufficient strength to do battle with the demon. This teaches us a great deal about holiness and also a great deal about evil. I am still unsure about the personification piece of this, and how it squares with Augustine's teaching on evil being an absence, but I do know that holiness, clinging to the sure hope in the Resurrection, is the only way to combat it at its root.