6 Faustus' argument is, If Elias who was a man could escape death, why might not Christ have the power of dying, since He was more than man? This is the same as to say, If human nature can be changed for the better, why should not the divine nature be changed for the worse?— a weak argument, seeing that human nature is changeable, while the divine nature is not. Such a method of inference would lead to the glaring absurdity, that if God can bestow eternal glory on man, He must also have the power of consigning Himself to eternal misery.
Faustus will reply that his argument refers only to three days of death for God, as compared with eternal life for man. Well, if you understood the three days of death in the sense of the death of the flesh which God took as a part of our mortal nature, you would be quite correct; for the truth of the gospel makes known that the death of Christ for three days was for the eternal life of men. But in arguing that there is no impropriety in asserting a death of three days of the divine nature itself, without any assumption of mortality, because human nature can be endowed with immortality, you display the folly of one who knows neither God nor the gifts of God.
And indeed, since you make part of your god to be fastened to the mass of darkness for ever, how can you escape the absurd conclusion already mentioned, that God consigns Himself to eternal misery? You will then require to prove that part of light is light, while part of God is not God. To give you in a word, without argument, the true reason of our faith, as regards Elias having been caught up to heaven from the earth, though only a man, and as regards Christ being truly born of a virgin, and truly dying on the cross, our belief in both cases is grounded on the declaration of Holy Scripture, which it is piety to believe, and impiety to disbelieve.
What is said of Elias you pretend to deny, for you will pretend anything. Regarding Christ, although even you do not go the length of saying that He could not die, though He could be born, still you deny His birth from a virgin, and assert His death on the cross to have been feigned, which is equivalent to denying it too, except as a mockery for the delusion of men; and you allow so much merely to obtain indulgence for your own falsehoods from the believers in these fictions.
Source: Reply to Faustus the Manichaean (New Advent)