After this, through the influence of some motive which is unknown to me, Celsus asserts that it is by the names of certain demons, and by the use of incantations, that the Christians appear to be possessed of (miraculous) power; hinting, I suppose, at the practices of those who expel evil spirits by incantations. And here he manifestly appears to malign the Gospel. For it is not by incantations that Christians seem to prevail (over evil spirits), but by the name of Jesus, accompanied by the announcement of the narratives which relate to Him; for the repetition of these has frequently been the means of driving demons out of men, especially when those who repeated them did so in a sound and genuinely believing spirit.
Such power, indeed, does the name of Jesus possess over evil spirits, that there have been instances where it was effectual, when it was pronounced even by bad men, which Jesus Himself taught (would be the case), when He said: “Many shall say to Me in that day, In Your name we have cast out devils, and done many wonderful works.” Whether Celsus omitted this from intentional malignity, or from ignorance, I do not know. And he next proceeds to bring a charge against the Saviour Himself, alleging that it was by means of sorcery that He was able to accomplish the wonders which He performed; and that foreseeing that others would attain the same knowledge, and do the same things, making a boast of doing them by help of the power of God, He excludes such from His kingdom.
And his accusation is, that if they are justly excluded, while He Himself is guilty of the same practices, He is a wicked man; but if He is not guilty of wickedness in doing such things, neither are they who do the same as He. But even if it be impossible to show by what power Jesus wrought these miracles, it is clear that Christians employ no spells or incantations, but the simple name of Jesus, and certain other words in which they repose faith, according to the holy Scriptures.
Source: Against Celsus (New Advent)