Concerning the Devil

And I said to him, “I should like to continue my questions.” “Speak on,” said he. And I said, “I heard, sir, some teachers maintain that there is no other repentance than that which takes place, when we descended into the water and received remission of our former sins.” He said to me, “That was sound doctrine which you heard; for that is really the case. For he who has received remission of his sins ought not to sin any more, but to live in purity. Since, however, you inquire diligently into all things, I will point this also out to you, not as giving occasion for error to those who are to believe, or have lately believed, in the Lord. For those who have now believed, and those who are to believe, have not repentance for their sins; but they have remission of their previous sins. For to those who have been called before these days, the Lord has set repentance. For the Lord, knowing the heart, and foreknowing all things, knew the weakness of men and the manifold wiles of the devil, that he would inflict some evil on the servants of God, and would act wickedly towards them. The Lord, therefore, being merciful, has had mercy on the work of His hand, and has set repentance for them; and He has entrusted to me power over this repentance. And therefore I say to you, that if any one is tempted by the devil, and sins after that great and holy calling in which the Lord has called His people to everlasting life, he has opportunity to repent but once. But if he should sin frequently after this, and then repent, to such a man his repentance will be of no avail; for with difficulty will he live.” And I said, “Sir, I feel that life has come back to me in listening attentively to these commandments; for I know that I shall be saved, if in future I sin no more.” And he said, “You will be saved, you and all who keep these commandments.”

─Hermas: The Pastor (or "The Shepherd") Bk. II, Commandment 12, Ch. 3.
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I say to him, “Sir, listen to me for a moment.” “Say what you wish,” says he. “Man, sir,” say I, “is eager to keep the commandments of God, and there is no one who does not ask of the Lord that strength may be given him for these commandments, and that he may be subject to them; but the devil is hard, and holds sway over them.” “He cannot,” says he, “hold sway over the servants of God, who with all their heart place their hopes in Him. The devil can wrestle against these, overthrow them he cannot. If, then, you resist him, he will be conquered, and flee in disgrace from you. As many, therefore,” says he, “as are empty, fear the devil, as possessing power. When a man has filled very suitable jars with good wine, and a few among those jars are left empty, then he comes to the jars, and does not look at the full jars, for he knows that they are full; but he looks at the empty, being afraid lest they have become sour. For empty jars quickly become sour, and the goodness of the wine is gone. So also the devil goes to all the servants of God to try them. As many, then, as are full in the faith, resist him strongly, and he withdraws from them, having no way by which he might enter them. He goes, then, to the empty, and finding a way of entrance, into them, he produces in them whatever he wishes, and they become his servants.” 

─Hermas: The Pastor, Bk. II, Commandment 12, Ch. 5
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Regarding the devil and his angels, and the opposing influences, the teaching of the Church has laid down that these beings exist indeed; but what they are, or how they exist, it has not explained with sufficient clearness. This opinion, however, is held by most, that the devil was an angel, and that, having become an apostate, he induced as many of the angels as possible to fall away with himself, and these up to the present time are called his angels.

─Origen: On First Principles, Preface, ("De Principiis," Proem.) 6.
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The devil’s snare does not catch you, unless you are first caught by the devil’s bait.

─St. Ambrose: Explanation of Psalm 118, 14, 37.
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The devil rules over lovers of temporal goods belonging to this visible world, not because he is lord of this world, but because he is ruler of those covetous desires by which we long for all that passes away.

─St. Augustine: The Christian combat ("De Agone Christiano"), Bk. 1, Ch. 1.
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For there is more than one way in which men sacrifice to the fallen angels.

─St. Augustine: Confessions, Bk. 1, Ch. 1 

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Fall of the Rebel Angels, by Domenico Beccafumi.
Oil on wood, c. 1528; San Niccolò al Carmine, Siena.