Celsus says that “the demons belong to God, and are therefore to be believed, to be sacrificed to according to laws, and to be prayed to that they may be propitious.” Those who are disposed to learn, must know that the word of God nowhere says of evil things that they belong to God, for it judges them unworthy of such a Lord. Accordingly, it is not all men who bear the name of “men of God,” but only those who are worthy of God—such as Moses and Elias, and any others who are so called, or such as resemble those who are so called in Scripture.
In the same way, all angels are not said to be angels of God, but only those that are blessed: those that have fallen away into sin are called “angels of the devil,” just as bad men are called “men of sin,” “sons of perdition,” or “sons of iniquity.” Since, then, among men some are good and others bad, and the former are said to be God's and the latter the devil's, so among angels some are angels of God, and others angels of the devil. But among demons there is no such distinction, for all are said to be wicked.
We do not therefore hesitate to say that Celsus is false when he says, “If they are demons, it is evident that they must also belong to God.” He must either show that this distinction of good and bad among angels and men has no foundation, or else that a similar distinction may be shown to hold among demons. If that is impossible, it is plain that demons do not belong to God; for their prince is not God, but, as holy Scripture says, “Beelzebub.”
Source: Against Celsus (New Advent)