In the next place, as if he had forgotten that it was his object to write against the Christians, he says that, “having become acquainted with one Dionysius, an Egyptian musician, the latter told him, with respect to magic arts, that it was only over the uneducated and men of corrupt morals that they had any power, while on philosophers they were unable to produce any effect, because they were careful to observe a healthy manner of life.” If, now, it had been our purpose to treat of magic, we could have added a few remarks in addition to what we have already said on this topic; but since it is only the more important matters which we have to notice in answer to Celsus, we shall say of magic, that any one who chooses to inquire whether philosophers were ever led captive by it or not, can read what has been written by Moiragenes regarding the memoirs of the magician and philosopher Apollonius of Tyana, in which this individual, who is not a Christian, but a philosopher, asserts that some philosophers of no mean note were won over by the magic power possessed by Apollonius, and resorted to him as a sorcerer; and among these, I think, he especially mentioned Euphrates and a certain Epicurean. Now we, on the other hand, affirm, and have learned by experience, that they who worship the God of all things in conformity with the Christianity which comes by Jesus, and who live according to His Gospel, using night and day, continuously and becomingly, the prescribed prayers, are not carried away either by magic or demons. For verily “the angel of the Lord encamps round about them that fear Him, and delivers them” from all evil; and the angels of the little ones in the Church, who are appointed to watch over them, are said always to behold the face of their Father who is in heaven, whatever be the meaning of “face” or of “behold.”
Source: Against Celsus (New Advent)