According to Sts. Diadochos of Photiki and Nikitas Stithatos, Satan and his demon-minions are “extremely frightened of the grace of the divine Spirit, especially when It is abundantly present in us or when we have been cleansed through spiritual reading and pure prayer” and, especially, the Sacrament of Reconciliation (Confession), because “as long as the Holy Spirit is in us, Satan cannot enter the depths of the soul and remain there.” The grace of the Holy Spirit in people, especially in Its fullness, fills Satan and his demons with terror “since they share in the divine fire and are in fact called fire.” This is why Satan and his demons will do anything to prevent us from reaching that state of theosis that is called deification.
In Met. Hierotheos, 2010, The science of spiritual medicine. Levadia, Greece: Birth of the Theotokos Monastery.