HOW MUCH POWER DOES THE DEVIL HAVE OVER US IN NORMAL CIRCUMSTANCES?

In general, the devil or any demon has no power to possess or enter a person who lives in a state of grace, especially a person who actively pursues a life of charity.  God's grace makes the soul into a spiritual stronghold that evil cannot penetrate save by the person's own free will allowing it to enter.  Demonic evil can only operate on a person with the consent of human will.

There are two exceptions to this general rule.  One is when God in His Providence allows a person in a state of grace to be afflicted by a demon.  The stories of God's permission for possession are not common in the history of the Church, but there are a few dramatic stories recounted by Fr. Amorth in his book, An Exorcist: More Stories, which tell of the possession of St. John Bosco, St. Gemma Galgani and Blessed Giovanni Calabria.  In each case, God allowed these saints to be possessed by the devil for their own sanctification; thus did God prove to the world that He can use the devil to accomplish His purposes.

The other exception to the rule is when an innocent person becomes the victim of a satanic curse or witchcraft of some sort.  Clearly there are innocent victims of occult evil, but even though the victims did nothing to bring the evil upon themselves through any personal consent, the evil act still involves the free will of some other human being cooperating with the powers of darkness.

By Fr. Thomas Euteneuer "Exorcism and the Church Militant" page 36.