Others, then, may concede to Celsus that God does not undergo a change, but leads the spectators to imagine that He does; whereas we who are persuaded that the advent of Jesus among men was no mere appearance, but a real manifestation, are not affected by this charge of Celsus. We nevertheless will attempt a reply, because you assert, Celsus, do you not, that it is sometimes allowable to employ deceit and falsehood by way, as it were, of medicine? Where, then, is the absurdity, if such a saving result were to be accomplished, that some such events should have taken place?
For certain words, when savouring of falsehood, produce upon such characters a corrective effect (like the similar declarations of physicians to their patients), rather than when spoken in the spirit of truth. This, however, must be our defence against other opponents. For there is no absurdity in Him who healed sick friends, healing the dear human race by means of such remedies as He would not employ preferentially, but only according to circumstances. The human race, moreover, when in a state of mental alienation, had to be cured by methods which the Word saw would aid in bringing back those so afflicted to a sound state of mind.
But Celsus says also, that “one acts thus towards enemies when taking measures to escape danger. But God does not fear any one, so as to escape danger by leading into error those who conspire against him.” Now it is altogether unnecessary and absurd to answer a charge which is advanced by no one against our Saviour. And we have already replied, when answering other charges, to the statement that “no one who is either in a state of sickness or mental alienation is a friend of God.”
For the answer is, that such arrangements have been made, not for the sake of those who, being already friends, afterwards fell sick or became afflicted with mental disease, but in order that those who were still enemies through sickness of the soul, and alienation of the natural reason, might become the friends of God. For it is distinctly stated that Jesus endured all things on behalf of sinners, that He might free them from sin, and convert them to righteousness.
Source: Against Celsus (New Advent)