It is therefore in vain that Celsus asserts, as one who knows not the nature of the Spirit of God, that “as the Son of God, who existed in a human body, is a Spirit, this very Son of God would not be immortal.” He next becomes confused in his statements, as if there were some of us who did not admit that God is a Spirit, but maintain that only with regard to His Son, and he thinks that he can answer us by saying that there “is no kind of spirit which lasts for ever.” This is much the same as if, when we term God a “consuming fire,” he were to say that there “is no kind of fire which lasts for ever;” not observing the sense in which we say that our God is a fire, and what the things are which He consumes, viz., sins, and wickedness.
For it becomes a God of goodness, after each individual has shown, by his efforts, what kind of combatant he has been, to consume vice by the fire of His chastisements. He proceeds, in the next place, to assume what we do not maintain, that “God must necessarily have given up the ghost;” from which also it follows that Jesus could not have risen again with His body. For God would not have received back the spirit which He had surrendered after it had been stained by contact with the body. It is foolish, however, for us to answer statements as ours which were never made by us.
Source: Against Celsus (New Advent)