After having said so much of the demons, and of their fondness for blood and the odour of sacrifices, Celsus adds, as though wishing to retract the charge he had made: “The more just opinion is, that demons desire nothing and need nothing, but that they take pleasure in those who discharge towards them offices of piety.” If Celsus believed this to be true, he should have said so, instead of making his previous statements. But, indeed, human nature is never utterly forsaken by God and His only-begotten Son, the Truth.
Wherefore even Celsus spoke the truth when he made the demons take pleasure in the blood and smoke of victims; although, by the force of his own evil nature, he falls back into his errors, and compares demons with men who rigorously discharge every duty, even to those who show no gratitude; while to those who are grateful they abound in acts of kindness. Here Celsus appears to me to get into confusion. At one time his judgment is darkened by the influence of demons, and at another he recovers from their deluding power, and gets some glimpses of the truth.
For again he adds: “We must never in any way lose our hold of God, whether by day or by night, whether in public or in secret, whether in word or in deed, but in whatever we do, or abstain from doing.” That is, as I understand it, whatever we do in public, in all our actions, in all our words, “let the soul be constantly fixed upon God.” And yet again, as though, after struggling in argument against the insane inspirations of demons, he were completely overcome by them, he adds: “If this is the case, what harm is there in gaining the favour of the rulers of the earth, whether of a nature different from ours, or human princes and kings?
For these have gained their dignity through the instrumentality of demons.” In a former part, Celsus did his utmost to debase our souls to the worship of demons; and now he wishes us to seek the favour of kings and princes, of whom, as the world and all history are full of them, I do not consider it necessary to quote examples.
Source: Against Celsus (New Advent)