30 Days: How does one become a victim of the demon?
Fr. Amorth: One can be subjected to the attacks of the demon in four cases.
1) Either because it constitutes a blessing for the person and such is the case with many of the saints (e.g. Saint Paul),
2) or because of irreversible persistence in sin,
3) or because one is the victim of a curse uttered in the name of the demon,
4) or else when one gives oneself up to practices of occultism.
Read the whole interview: Fr. Amorth Interview with 30 Days