17 Therefore, my beloved, when the Holy Spirit departs from a man who has received it, until it returns and comes to him, then Satan draws near unto that man, to cause him to sin, and that the Holy Spirit may leave him altogether. For as long as the Spirit is with a man, Satan fears to come near him. And observe, my beloved, that our Lord also, Who was born from the Spirit, was not tempted by Satan until in baptism He received the Spirit from on high. And then the Spirit led him forth to be tempted by Satan.
This, then, is the way with man; that in the hour in which he perceives in himself that he is not fervent in the Spirit, and that his heart is inclining to the thought of this world, he may know that the Spirit is not with him, and may arise and pray and keep vigil that the Spirit of God may come to him, that he be not overcome by the adversary. A thief does not dig into a house, until he sees that its master is departing from it. Thus also Satan cannot draw near to that house which is our body, until the Spirit of Christ departs from it.
And be sure, my beloved, that the thief does not certainly know whether the master of the house is within or not, but first he applies his ear, and looks. If he hears the voice of the master of the house within it saying:— “I have a journey to go,” and when he has searched out and seen that the master of the house has set out to perform his business, then the thief comes and digs into the house and steals. But if he hears the voice of the master of the house admonishing and commanding his household to watch and guard his house, and saying to them, “I also am within the house,” then the thief will fear and flee, that he may not be taken and captured.
Thus also Satan, he has not the knowledge beforehand to know or see when the Spirit will depart, that so he may come to rob the man; but he too listens and watches, and so assails. But if he hears a man in whom Christ dwells speaking shameful words, or enraged, or quarrelling, or contending, then Satan knows that Christ is not with him, and he comes and accomplishes his will in him. For Christ dwells in the peaceful and the meek, and lodges in those that fear His word, as He says through the prophet:— On whom shall I look, and in whom shall I dwell, but in the peaceful and the meek who fear My word? And our Lord said:— Whoever walks in My commandments and keeps My love, We will come to him and make Our abode with him. But if he hears from a man that he is on his guard and is praying and meditating in the Law of his Lord by day and by night, then he turns back from him, for he knows that Christ is with him.
And if you should say, “How manifold is Satan! For lo! He fights with many;” then hear and learn from that which I proved to you above concerning Christ, that no matter to what extent He is divided among many, yet He is not a whit diminished. For, as the house, through the window of which a little sunlight enters, is altogether illumined, so the man into whom a little of Satan enters, is altogether darkened. Hear that which the Apostle said:— If Satan is transfigured to an angel of light, it is no wonder if his ministers also are transfigured to ministers of righteousness. And again our Lord said to His disciples:— Lo, I have given you authority to tread upon the power of the adversary. And the Scriptures have made known that he has power and also ministers.
Moreover Job said concerning him:— God made him to wage his war. These ministers then that he has, he causes to run in the world, to wage war. But be sure that he will not fight openly; because from the time of the coming of our Saviour, (God) has given authority over him. But he will surely plunder and steal.
Source: Demonstrations (New Advent)