And he continues: “What is the meaning of such a descent upon the part of God?” not observing that, according to our teaching, the meaning of the descent is pre-eminently to convert what are called in the Gospel the lost “sheep of the house of Israel;” and secondly, to take away from them, on account of their disobedience, what is called the “kingdom of God,” and to give to other husbandmen than the ancient Jews, viz. to the Christians, who will render to God the fruits of His kingdom in due season (each action being a “fruit of the kingdom”). We shall therefore, out of a greater number, select a few remarks by way of answer to the question of Celsus, when he says, “What is the meaning of such a descent upon the part of God?”
And Celsus here returns to himself an answer which would have been given neither by Jews nor by us, when he asks, “Was it in order to learn what goes on among men?” For not one of us asserts that it was in order to learn what goes on among men that Christ entered into this life. Immediately after, however, as if some would reply that it was “in order to learn what goes on among men,” he makes this objection to his own statement: “Does he not know all things?” Then, as if we were to answer that He does know all things, he raises a new question, saying, “Then he does know, but does not make (men) better, nor is it possible for him by means of his divine power to make (men) better.”
Now all this on his part is silly talk; for God, by means of His word, which is continually passing from generation to generation into holy souls, and constituting them friends of God and prophets, does improve those who listen to His words; and by the coming of Christ He improves, through the doctrine of Christianity, not those who are unwilling, but those who have chosen the better life, and that which is pleasing to God. I do not know, moreover, what kind of improvement Celsus wished to take place when he raised the objection, asking, “Is it then not possible for him, by means of his divine power, to make (men) better, unless he send some one for that special purpose?” Would he then have the improvement to take place by God's filling the minds of men with new ideas, removing at once the (inherent) wickedness, and implanting virtue (in its stead)? Another person now would inquire whether this was not inconsistent or impossible in the very nature of things; we, however, would say, “Grant it to be so, and let it be possible.”
Where, then, is our free will? and what credit is there in assenting to the truth? Or how is the rejection of what is false praiseworthy? But even if it were once granted that such a course was not only possible, but could be accomplished with propriety (by God), why would not one rather inquire (asking a question like that of Celsus) why it was not possible for God, by means of His divine power, to create men who needed no improvement, but who were of themselves virtuous and perfect, evil being altogether non-existent?
These questions may perplex ignorant and foolish individuals, but not him who sees into the nature of things; for if you take away the spontaneity of virtue, you destroy its essence. But it would need an entire treatise to discuss these matters; and on this subject the Greeks have expressed themselves at great length in their works on providence. They truly would not say what Celsus has expressed in words, that “God knows (all things) indeed, but does not make (men) better, nor is able to do so by His divine power.” We ourselves have spoken in many parts of our writings on these points to the best of our ability, and the Holy Scriptures have established the same to those who are able to understand them.
Source: Against Celsus (New Advent)