Question Four of Chapter Three of the Third Decade
That a demon cannot enter the soul
It is asked whether demons can enter the souls of human beings.
To this it first appears that one must respond affirmatively, through that saying of John, chapter 13, where it is related that after the morsel of bread was offered by Christ to Judas, Satan entered into him.
And in John, chapter 13: “The devil put it into the heart that Judas should betray Christ.”
In response it must be said that Satan’s entering Judas or another human being can be understood in two ways.
In one way, he enters the human body, as in those who are bodily vexed by a demon.
In this manner, as we said a little before, the conclusion is that a demon can enter a human being through its essence. For, on account of the subtlety of its nature, a demon can penetrate bodies without any obstacle and, moreover, molest and disturb them, as is clearly evident in the Gospels concerning those whom Christ drove out from bodies in Matthew, chapter 9, and Luke, chapter 8.
In another way, it can be understood that the devil enters the mind in such a manner that the devil enters the mind through his essence. In this manner the conclusion is that the devil cannot enter the mind of a human being; only God can do so.
The reason is that, since the rational soul is simple and incorporeal, it does not possess dimensions of quantity such that something could be said to be in it as though contained beneath its dimensions. Therefore nothing can be in it except that which gives it being and which is in it through its own power.
I say, therefore, that God alone, who brought the soul into being, is in the soul through his power.
But where the power of God is, there is the essence of God, since in God power and essence are the same.
Therefore God alone is in the soul through his essence, and he alone transfers himself into it through grace.
To the argument it is said that the devil is said to have entered the heart or mind of Judas or of a human being through the affection or effect of malice—insofar, namely, as a human being seduced by him follows him in perpetrating the evil which he suggests. In this manner he is said to have entered the heart of Judas.
Hence Nicholas of Lyra says upon that passage of Luke, chapter 22, “Satan entered Judas”: that is, he entered the soul not through an entering-in, because this is proper to God, but through an effect—namely, by suggesting to him the sale of Christ.
And Albert says upon Luke: “After the morsel Satan entered Judas,” that is, in order to possess him more fully—not, however, because he entered his mind or soul, but because, through suggestion, he found in him a place for his malice.
These things are from Thomas in the Quodlibets and upon John; chapter Sicut Iudas, On Consecration, distinction 2; and Lord Johannes de Turrecremata, Cardinal, in his exposition of the Gospels.
Addition
Although demons can enter human bodies unless they are prohibited by God, nevertheless they cannot enter souls, either immediately by themselves or even through angels as intermediaries, nor can they penetrate the highest or innermost part of the soul.
Thus Saint Augustine writes in chapter 50 of On Ecclesiastical Dogmas.
And Bede, upon that saying of Acts, chapter 5, “Ananias, why has Satan tempted your heart?”, says: “Not indeed entering into him and into his sense, nor entering the approach to the heart, since this power belongs to God alone, but only moving his heart.”