And it is not we alone who speak of wicked demons, but almost all who acknowledge the existence of demons. Thus, then, it is not true that all observe the law of the Most High; for all who fall away from the divine law, whether through heedlessness, or through depravity and vice, or through ignorance of what is right, all such do not keep the law of God, but, to use a new phrase which we find in Scripture, “the law of sin.” I say, then, that in the opinion of most of those who believe in the existence of demons, some of them are wicked; and these, instead of keeping the law of God, offend against it.
But, according to our belief, it is true of all demons, that they were not demons originally, but they became so in departing from the true way; so that the name “demons” is given to those beings who have fallen away from God. Accordingly, those who worship God must not serve demons. We may also learn the true nature of demons if we consider the practice of those who call upon them by charms to prevent certain things, or for many other purposes. For this is the method they adopt, in order by means of incantations and magical arts to invoke the demons, and induce them to further their wishes.
Wherefore, the worship of all demons would be inconsistent in us who worship the Supreme God; and the service of demons is the service of so-called gods, for “all the gods of the heathen are demons.” The same thing also appears from the fact that the dedication of the most famous of the so-called sacred places, whether temples or statues, was accompanied by curious magical incantations, which were performed by those who zealously served the demons with magical arts. Hence we are determined to avoid the worship of demons even as we would avoid death; and we hold that the worship, which is supposed among the Greeks to be rendered to gods at the altars, and images, and temples, is in reality offered to demons.
Source: Against Celsus (New Advent)