St. Teresa of Avila was a 16th century Spanish mystic and is honored today as a Doctor of the Church for her incredible insight into the spiritual life. And in her prayers and meditations, she regularly came in contact with the demonic.
“An abominable form,” she writes of how the devil appeared to her, “his mouth was horrible. Out of his body there seemed to be coming a great flame, which cast no shadow.”
Most incredibly, she once saw a priest attacked by demons while he was saying Mass: “with the eyes of the soul [she saw] two devils of hideous aspect who seemed to have their horns around a priest’s throat while he celebrated Mass.”
Yet, even for her, these visual manifestations were relatively rare. “I have seldom seen him in bodily shape,” she writes, “but I have often seen him without any form, as in the kind of vision I have described, in which no form is seen but the object is known to be there.”
Her weapons against these evil forces? Prayer, humility, and – interestingly enough – holy water, which she claimed from experience was a particularly effective spiritual weapon.