Acknowledging the existence of the devil is essential, the pontiff insisted, because the spiritual warfare Christians fight is not merely “a battle against the world and a worldly mentality” or a “struggle against our human weaknesses and proclivities.”

“It is also a constant struggle against the devil, the prince of evil,” he said. The last chapter is given to spiritual combat, and he reminds the faithful that the Christian life “is a constant battle,” but adds that this battle “is sweet, for it allows us to rejoice each time the Lord triumphs in our lives.” He also stresses that this battle is not just against “the world and a worldly mentality” or “human weakness,” but “a constant struggle against the devil.”

The devil is not “a myth,” he says, adding that he “does not need to possess us,” but simply “poisons us with the venom of hatred, desolation, envy and vice.” Those who fail to realize it is a constant battle “will be prey to failure or mediocrity,” he adds, stressing that the Lord has given us “powerful weapons” to fight the devil such as “faith-filled prayer, meditation on the word of God, the celebration of Mass, Eucharistic adoration, sacramental reconciliation, works of charity, community life, missionary outreach.”

He goes on to warn against “spiritual corruption,” which he describes as a “comfortable and self-satisfied form of blindness” where all appears acceptable: “deception, slander, egotism and other subtle forms of self-centredness, for even Satan disguises himself as an angel of light.”

Pope Francis ends by returning to his common theme of discernment, which he says is “something more than intelligence or common sense.” It is a gift, he says, which we must implore and seek to develop through prayer, reflection, reading and good counsel.
In a new apostolic exhortation, Gaudete et Exsultate, Pope Francis mentions Satan or the Devil twelve times. Warning against Catholic media transgressing the eighth commandment, he calls to "see how the unguarded tongue, set on fire by hell, sets all things ablaze."