Unbelief the Demon's Stronghold.
"But it is necessary in our prayers to acknowledge that we have had recourse to God, and to bear witness, not to the apathy, but to the slowness of the demon. For all things are done to the believer, nothing to the unbeliever. Therefore the demons themselves, knowing the amount of faith of those of whom they take possession, measure their stay proportionately. Wherefore they stay permanently with the unbelieving, tarry for a while with the weak in faith; but with those who thoroughly believe, and who do good, they cannot remain even for a moment. For the soul being turned by faith, as it were, into the nature of water, quenches the demon as a spark of fire. The labour, therefore, of every one is to be solicitous about the putting to flight of his own demon. For, being mixed up with men's souls, they suggest to every one's mind desires after what things they please, in order that he may neglect his salvation.