For consider with yourself which disposition of mind will be more acceptable to the Most High, whose power is supreme and universal, and who directs all for the welfare of mankind in body, and in mind, and in outward things—whether that of the man who gives himself up to God in all things, or that of the man who is curiously inquisitive about the names of demons, their powers and agency, the incantations, the herbs proper to them, and the stones with the inscriptions graven on them, corresponding symbolically or otherwise to their traditional shapes?
It is plain even to the least intelligent, that the disposition of the man who is simpleminded and not given to curious inquiries, but in all things devoted to the divine will, will be most pleasing to God, and to all those who are like God; but that of the man who, for the sake of bodily health, of bodily enjoyment, and outward prosperity, busies himself about the names of demons, and inquires by what incantations he shall appease them, will be condemned by God as bad and impious, and more agreeable to the nature of demons than of men, and will be given over to be torn and otherwise tormented by demons.
For it is probable that they, as being wicked creatures, and, as Celsus confesses, addicted to blood, sacrificial odours, sweet sounds, and such like, will not keep their most solemn promises to those who supply them with these things. For if others invoke their aid against the persons who have already called upon them, and purchase their favour with a larger supply of blood, and odours, and such offerings as they require, they will take part against those who yesterday sacrificed and presented pleasant offerings to them.
Source: Against Celsus (New Advent)