For it is impossible that the good which is the result of accident, or of communication, should be like that good which comes by nature; and yet the former will never be lost by him who, so to speak, partakes of the “living” bread with a view to his own preservation. But if it should fail any one, it must be through his own fault, in being slothful to partake of this “living bread” and “genuine drink,” by means of which the wings, nourished and watered, are fitted for their purpose, even according to the saying of Solomon, the wisest of men, concerning the truly rich man, that “he made to himself wings like an eagle, and returns to the house of his patron.” For it became God, who knows how to turn to proper account even those who in their wickedness have apostatized from Him, to place wickedness of this sort in some part of the universe, and to appoint a training-school of virtue, wherein those must exercise themselves who would desire to recover in a “lawful manner” the possession (which they had lost); in order that being tested, like gold in the fire, by the wickedness of these, and having exerted themselves to the utmost to prevent anything base injuring their rational nature, they may appear deserving of an ascent to divine things, and may be elevated by the Word to the blessedness which is above all things, and so to speak, to the very summit of goodness. Now he who in the Hebrew language is named Satan, and by some Satanas— as being more in conformity with the genius of the Greek language— signifies, when translated into Greek, “adversary.” But every one who prefers vice and a vicious life, is (because acting in a manner contrary to virtue) Satanas, that is, an “adversary” to the Son of God, who is righteousness, and truth, and wisdom. With more propriety, however, is he called “adversary,” who was the first among those that were living a peaceful and happy life to lose his wings, and to fall from blessedness; he who, according to Ezekiel, walked faultlessly in all his ways, “until iniquity was found in him,” and who being the “seal of resemblance” and the “crown of beauty” in the paradise of God, being filled as it were with good things, fell into destruction, in accordance with the word which said to him in a mystic sense: “You have fallen into destruction, and shall not abide for ever.” We have ventured somewhat rashly to make these few remarks, although in so doing we have added nothing of importance to this treatise. If any one, however, who has leisure for the examination of the sacred writings, should collect together from all sources and form into one body of doctrine what is recorded concerning the origin of evil, and the manner of its dissolution, he would see that the views of Moses and the prophets regarding Satan had not been even dreamed of either by Celsus or any one of those whose soul had been dragged down, and torn away from God, and from right views of Him, and from His word, by this wicked demon.
Source: Against Celsus (New Advent)