His next remark upon the Christians is: “They will admit that these images, whether they are like or not, are made and dedicated to the honour of certain beings; but they will hold that the beings to whom they are dedicated are not gods, but demons, and that a worshipper of God ought not to worship demons.” If he had been acquainted with the nature of demons, and with their several operations, whether led on to them by the conjurations of those who are skilled in the art, or urged on by their own inclination to act according to their power and inclination; if, I say, he had thoroughly understood this subject, which is both wide in extent and difficult for human comprehension, he would not have condemned us for saying that those who worship the Supreme Being should not serve demons. For ourselves, so far are we from wishing to serve demons, that by the use of prayers and other means which we learn from Scripture, we drive them out of the souls of men, out of places where they have established themselves, and even sometimes from the bodies of animals; for even these creatures often suffer from injuries inflicted upon them by demons.
Source: Against Celsus (New Advent)