And we affirm indeed the existence of certain spiritual essences; nor is their name unfamiliar. The philosophers acknowledge there are demons; Socrates himself waiting on a demon's will. Why not? Since it is said an evil spirit attached itself specially to him even from his childhood— turning his mind no doubt from what was good. The poets are all acquainted with demons too; even the ignorant common people make frequent use of them in cursing. In fact, they call upon Satan, the demon-chief, in their execrations, as though from some instinctive soul-knowledge of him.
Plato also admits the existence of angels. The dealers in magic, no less, come forward as witnesses to the existence of both kinds of spirits. We are instructed, moreover, by our sacred books how from certain angels, who fell of their own free-will, there sprang a more wicked demon-brood, condemned of God along with the authors of their race, and that chief we have referred to. It will for the present be enough, however, that some account is given of their work. Their great business is the ruin of mankind.
So, from the very first, spiritual wickedness sought our destruction. They inflict, accordingly, upon our bodies diseases and other grievous calamities, while by violent assaults they hurry the soul into sudden and extraordinary excesses. Their marvellous subtleness and tenuity give them access to both parts of our nature. As spiritual, they can do no harm; for, invisible and intangible, we are not cognizant of their action save by its effects, as when some inexplicable, unseen poison in the breeze blights the apples and the grain while in the flower, or kills them in the bud, or destroys them when they have reached maturity; as though by the tainted atmosphere in some unknown way spreading abroad its pestilential exhalations.
So, too, by an influence equally obscure, demons and angels breathe into the soul, and rouse up its corruptions with furious passions and vile excesses; or with cruel lusts accompanied by various errors, of which the worst is that by which these deities are commended to the favour of deceived and deluded human beings, that they may get their proper food of flesh-fumes and blood when that is offered up to idol-images. What is daintier food to the spirit of evil, than turning men's minds away from the true God by the illusions of a false divination?
And here I explain how these illusions are managed. Every spirit is possessed of wings. This is a common property of both angels and demons. So they are everywhere in a single moment; the whole world is as one place to them; all that is done over the whole extent of it, it is as easy for them to know as to report. Their swiftness of motion is taken for divinity, because their nature is unknown. Thus they would have themselves thought sometimes the authors of the things which they announce; and sometimes, no doubt, the bad things are their doing, never the good.
The purposes of God, too, they took up of old from the lips of the prophets, even as they spoke them; and they gather them still from their works, when they hear them read aloud. Thus getting, too, from this source some intimations of the future, they set themselves up as rivals of the true God, while they steal His divinations. But the skill with which their responses are shaped to meet events, your Crœsi and Pyrrhi know too well. On the other hand, it was in that way we have explained, the Pythian was able to declare that they were cooking a tortoise with the flesh of a lamb; in a moment he had been to Lydia.
From dwelling in the air, and their nearness to the stars, and their commerce with the clouds, they have means of knowing the preparatory processes going on in these upper regions, and thus can give promise of the rains which they already feel. Very kind too, no doubt, they are in regard to the healing of diseases. For, first of all, they make you ill; then, to get a miracle out of it, they command the application of remedies either altogether new, or contrary to those in use, and straightway withdrawing hurtful influence, they are supposed to have wrought a cure.
What need, then, to speak of their other artifices, or yet further of the deceptive power which they have as spirits: of these Castor apparitions, of water carried by a sieve, and a ship drawn along by a girdle, and a beard reddened by a touch, all done with the one object of showing that men should believe in the deity of stones, and not seek after the only true God?
Source: The Apology (New Advent)