Of the fact that devils struggle with men not without effort on their part.
But our belief is that they undertake this struggle not without effort on their part. For in their conflict they themselves have some sort of anxiety and depression, and especially when they are matched with stronger rivals, i.e., saints and perfect men. Otherwise no contest or struggle, but only a simple deception of men, and one free from anxiety on their part would be assigned to them. And how then would the Apostle's words stand, where he says: “We wrestle not against flesh and blood, but against principalities, against powers, against world rulers of this darkness, against spiritual wickedness in heavenly places;” and this too: “So fight I, not as one that beats the air;” and again: “I have fought a good fight”? For where it is spoken of as a fight, and conflict, and battle, there must be effort and exertion and anxiety on both sides, and equally there must either be in store for them chagrin and confusion for their failure, or delight consequent upon their victory. But where one fights with ease and security against another who struggles with great effort, and in order to overthrow his rival makes use of his will alone as his strength, there it ought not to be called a battle, struggle, or strife, but a sort of unfair and unreasonable assault and attack. But they certainly have to labour, and when they attack men, exert themselves in no lesser degree in order to secure from each one that victory which they want to obtain, and there is hurled back upon them the same confusion which was awaiting us had we been worsted by them; as it is said: “The head of their compassing me about, the labour of their own lips shall overwhelm them;” and: “His sorrow shall be turned on his own head;” and again: “Let the snare which he knows not come upon him, and let the net which he has hidden catch him, and into that very snare let him fall;” viz., that which he contrived for the deception of men. They then themselves also come to grief, and as they damage us so are they also in like manner damaged by us, nor when they are worsted do they depart without confusion, and seeing these defeats of theirs and their struggles, one who had good eyes in his inner man, seeing also that they gloated over the downfall and mischances of individuals, and fearing lest his own case might furnish them with this kind of delight, prayed to the Lord saying: “Lighten my eyes that I sleep not in death: lest mine enemy say, I have prevailed against him. They that trouble me will rejoice if I be moved;” and: “O My God, let them not rejoice over me; let them not say in their hearts, Aha, Aha, our very wish; neither let them say; we have devoured him;” and: “They gnashed their teeth upon me. Lord, how long will You look on this?” for: “he lies in wait secretly as a lion in his den: he lies in wait to ravish the poor;” and: “He seeks from God his meat.” And again when all their efforts are exhausted, and they have failed to secure our deception, they must “be confounded and blush” at the failure of their efforts, “who seek our souls to destroy them: and let them be covered with shame and confusion who imagine evil against us.” Jeremiah also says: “Let them be confounded, and let not me be confounded: let them be afraid, and let not me be afraid: bring upon them the fury of Your wrath, and with a double destruction destroy them.” For no one can doubt that when they are vanquished by us they will be destroyed with a double destruction: first, because while men are seeking after holiness, they, though they possessed it, lost it, and became the cause of man's ruin; secondly, because being spiritual existences, they have been vanquished by carnal and earthly ones. Each one then of the saints when he looks on the destruction of his foes and his own triumphs, exclaims with delight: “I will follow after mine enemies and overtake them: and I will not turn until they are destroyed. I will break them and they shall not be able to stand: they shall fall under my feet,” and in his prayers against them the same prophet says: “Judge, O Lord, them that wrong me: overthrow them that fight against me. Take hold of arms and shield: and rise up to help me. Bring out the sword and shut up the way against them that persecute me: say to my soul, I am your salvation.” And when by subduing and destroying all our passions we have vanquished these, we shall then be permitted to hear those words of blessing: “Your hand shall be exalted over your enemies, and all your enemies shall perish.” And so when we read or chant all these and such like passages found in holy writ, unless we take them as written against those spiritual wickednesses which lie in wait for us night and day, we shall not only fail to draw from them any edification to make us gentle and patient, but shall actually meet with some dreadful consequence and one that is quite contrary to evangelical perfection. For we shall not only not be taught to pray for or to love our enemies, but actually shall be stirred up to hate them with an implacable hatred, and to curse them and incessantly to pour forth prayers against them. And it is terribly wrong and blasphemous to think that these words were uttered in such a spirit by holy men and friends of God, on whom before the coming of Christ the law was not imposed for the very reason that they went beyond its commands, and chose rather to obey the precepts of the gospel and to aim at perfection, though they lived before the dispensation of the time.
On the fact that the power to hurt does not depend upon the will of the devils.
But that they have not the power of hurting any man is shown in a very clear way by the instance of the blessed Job, where the enemy did not venture to try him beyond what was allowed to him by the Divine permission; and it is evidenced by the confession of the same spirits contained in the records of the gospel, where they say: “If You cast us out, suffer us to go into the herd of swine.” Matthew 8:31 And far more must we hold that they cannot of their own free will enter into any one of men who are created in the image of God, if they have not power to enter into dumb and unclean animals without the permission of God. But no one— I will not say of the younger men, whom we see living most steadfastly in this desert, but even of those who are perfect— could live alone in the desert, surrounded by such swarms of foes of this kind, if they had unlimited power and freedom to hurt and tempt us: and still more clearly is this supported by the words of our Lord and Saviour, which in the lowliness of the manhood He had assumed, He uttered to Pilate, when He said: “You could have no power against Me at all, unless it were given you from above.”
Source: Conferences (New Advent)