Celsus, however, suspecting that the tendency of such teaching as he here gives is to lead to magic, and dreading that harm may arise from these statements, adds: “Care, however, must be taken lest any one, by familiarizing his mind with these matters, should become too much engrossed with them, and lest, through an excessive regard for the body, he should have his mind turned away from higher things, and allow them to pass into oblivion. For perhaps we ought not to despise the opinion of those wise men who say that most of the earth-demons are taken up with carnal indulgence, blood, odours, sweet sounds, and other such sensual things; and therefore they are unable to do more than heal the body, or foretell the fortunes of men and cities, and do other such things as relate to this mortal life.”
If there is, then, such a dangerous tendency in this direction, as even the enemy of the truth of God confesses, how much better is it to avoid all danger of giving ourselves too much up to the power of such demons, and of becoming turned aside from higher things, and suffering them to pass into oblivion through an excessive attention to the body; by entrusting ourselves to the Supreme God through Jesus Christ, who has given us such instruction, and asking of Him all help, and the guardianship of holy and good angels, to defend us from the earth-spirits intent on lust, and blood, and sacrificial odours, and strange sounds, and other sensual things!
For even, by the confession of Celsus, they can do nothing more than cure the body. But, indeed, I would say that it is not clear that these demons, however much they are reverenced, can even cure the body. But in seeking recovery from disease, a man must either follow the more ordinary and simple method, and have recourse to medical art; or if he would go beyond the common methods adopted by men, he must rise to the higher and better way of seeking the blessing of Him who is God over all, through piety and prayers.
Source: Against Celsus (New Advent)