As Mother Teresa lay in a Calcutta hospital the year before she died, her violently disturbed sleep became intolerable, her doctors seemingly unable to help. The problem, the visiting Archbishop of Calcutta suggested, was not a medical matter but something altogether more sinister: this living saint was being attacked by the devil. It was only after an exorcist was called that peace was restored.

That a woman regarded as so holy could have been subject to diabolical visitations was reported with surprise. Yet, if a course for Catholic clergy that took place in Rome last week is anything to go by, exorcism is far from rare. The programme trains about 200 priests a year in the ancient rite. For outsiders, exorcism appears anachronistic, a throwback to the superstitions of the middle ages. What the scale of last week's course shows, however, is that the devil is alive and kicking in Pope Francis's apparently modernising church.

In recent decades, the church has been surprisingly vocal on the issue. In 1975, the former Roman Inquisition published a study called Christian Faith and Demonology, with the aim of making the reality of the devil clear. Three years earlier, Pope Paul VI – surely a man of the modern age, given his 1968 encyclical prompted by the contraceptive pill and the miniskirt – said evil "is a living spiritual being, perverted and corrupting" and certainly not "a conceptual and imaginary personification of the unknown causes of our ills". Just last Tuesday, Francis himself put great emphasis on the role of the devil when speaking of the protomartyr Saint Stephen, saying that the "struggle between God and the devil" was apparent in the persecution of the church's people. For the hierarchy, the devil is not to be forgotten nor softened into a metaphor.

Meanwhile, modern holy people have kept the devil alive in the Catholic popular imagination. The 20th-century saint Padre Pio encountered him as a smoke-breathing dog and a naked dancing girl, also reporting being dragged from his bed by demonic forces. And the French mystic Marthe Robin, who died only in 1981, apparently lost two teeth as a result of the devil punching her in the face.

The devil continues to be as useful for the modern church as he has been in the past, when he bolstered the case for the burning of heretics. The concept now provides a dramatic way to underscore the dangers of a godless society. The organiser of last week's course, Dr Giuseppe Ferrari, argues that a rise in the number of people abandoning religion and dabbling in the occult has increased Satan's power. As head of the Gruppo di Ricerca e Informazione Socio-Religiosa, a Catholic organisation concerned with the threat posed by cults and sects, Ferrari says good exorcists are needed more than ever, since: "We live in a disenchanted society, a secularised world that thought it was being emancipated, but where religion is being thrown out, the window is being opened to superstition and irrationality."

This seems like an extreme position, but it is in perfect alignment with Francis's views, which go further than his brief mentions of the devil last week suggest. In his very first homily as pope, delivered in the Sistine Chapel on the day after his election, Francis bluntly quoted the French author and Catholic convert Léon Bloy: "Anyone who does not pray to the Lord prays to the devil."

In its biblical guises – Lucifer as chief of the fallen angels, the dragon of Revelation, the serpent in the Garden of Eden, or the tempter of Jesus in the wilderness – the idea of a supreme embodiment of evil is one that will always endure. And in the church's battle against secularism, the devil may have found a valuable new role.