"It is also necessary to know one very important thing that Saint John of the Cross and the great masters of the spiritual life have always said: the demon has no access to a person's heart, to a person's inmost being, to the soul; he can attack only through the senses. This is what we see in our own temptations: they always involve pride, vanity, gluttony, lack of chastity, hardheartedness, everything that comes to us from our sensibility; this is the field in which the Tempter operates. The soul is in God's hands, and only God has access to the holy of holies, to that deepest, most intimate part of the human person.
... The demon is a reality, but it is not a reality that should obsess us. In the Catechism of the Catholic Church we read: 'The power of Satan is ... not infinite. He is only a creature, powerful from the fact that he is pure spirit, but still a creature. He cannot prevent the building up of God's reign. Although Satan may act in the world out of hatred for God and his kingdom in Christ Jesus, and although his action may cause grave injuries - of a spiritual nature and, indirectly, even of a physical nature - to each man and to society, the action is permitted by divine providence which with strength and gentleness guides human and cosmic history. It is a great mystery that providence should permit diabolical activity, but 'we know what in everything God works for good with those who love him' (Rom 8:28)." CCC par. 395