“This kind goeth not out but by prayer and fasting.” [Matthew 17:14-23]
If this kind goes out by the prayer and fasting of another person, then it is even less able to enter one who fasts and prays.
Although there are a slew of demons and all the air is packed with them, they cannot do anything to one who is protected by prayer and fasting.
Fasting is universal temperance, prayer is universal communication with God; the former defends from the outside, whereas the latter from within directs a fiery weapon against the enemies. The demons can sense a faster and man of prayer from a distance, and they run far away from him so as avoid a painful blow.
Is it feasible to think that where there is no fasting and prayer, there already is a demon? Yes, it is.
The demons lodging in a person, do not always reveal their presence, but lurk there, stealthily teaching their host every evil and turning him away from every good thing; so this person is certain that he is doing everything on his own, but meanwhile he is only fulfilling the will of his enemy.
Just commence prayer and fasting and the enemy will immediately depart, then wait on the side for an opportunity to somehow return again. And he truly will return, as soon as prayer and fasting are abandoned.
+ St. Theophan the Recluse, Thoughts for Each Day of the Year: According to the Daily Church Readings from the Word of God