Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter XXIV. Of the way in which the devils prepare for themselves an entrance into the bodies of those whom they are going to possess.

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Chapter XXIV.

Of the way in which the devils prepare for themselves an entrance into the bodies of those whom they are going to possess.

It is clear then that unclean spirits cannot make their way into those whose bodies they are going to seize upon, in any other way than by first taking possession of their minds and thoughts. And when they have robbed them of fear and the recollection of God and spiritual meditation, they boldly advance upon them, as if they were dispossessed of all protection and Divine safeguard, and could easily be bound, and then take up their dwelling in them as if in a possession given over to them.