Paul VI in 1972

Evil is not merely an absence of something but an active force, a living, spiritual being that is perverted and that perverts others. It is a terrible reality, mysterious and frightening.
... He undermines man's moral equilibrium with his sophistry. He is the malign, clever seducer who knows how to make his way into us through the senses, the imagination and the libido, through utopian logic, or through disordered social contacts in the give and take of our activities, so that he can bring about in us deviations that are all the more harmful because they seem to conform to our physical or mental makeup, or to our profound, instinctive aspirations.
... Our doctrine becomes uncertain, darkness obscured as it is by the darkness surrounding the Devil. But our curiosity, excited by the certainty of his multiple existence, has a right to raise two questions. Are there signs, and what are they, of the presence of diabolical action? And what means of defense do we have against such an insidious danger?
... We can presume that his sinister action is at work where the denial of God becomes radical, subtle and absurd; where lies become powerful and hypocritical in the face of evident truth; where love is smothered by cold, cruel selfishness; where Christ's name is attacked with conscious, rebellious hatred, where the spirit of the Gospel is watered down and rejected; where despair is affirmed as the last word; and so forth.
... Grace is the decisive defense. Innocence takes on the aspect of strength. Everyone recalls how often the apostolic method of teaching used the armor of a soldier as a symbol for the virtues that can make a Christian invulnerable. The Christian must be a militant; he must be vigilant and strong; and he must at times make use of special ascetical practices to escape from certain diabolical attacks. Jesus teaches us this by pointing to "prayer and fasting" as the remedy. And the Apostle suggests the main line we should follow:

"Be not overcome by evil, but overcome evil with good."

[from EWTN]