He proceeds to repeat himself, and after saying a great deal which he had said before, and ridiculing the birth of God from a virgin—to which we have already replied as we best could—he adds the following: “If God had wished to send down His Spirit from Himself, what need was there to breathe it into the womb of a woman? For as one who knew already how to form men, He could also have fashioned a body for this person, without casting His own Spirit into so much pollution; and in this way He would not have been received with incredulity, if He had derived His existence immediately from above.”
He had made these remarks, because he knows not the pure and virgin birth, unaccompanied by any corruption, of that body which was to minister to the salvation of men. For, quoting the sayings of the Stoics, and affecting not to know the doctrine about “things indifferent,” he thinks that the divine nature was cast amid pollution, and was stained either by being in the body of a woman, until a body was formed around it, or by assuming a body. And in this he acts like those who imagine that the sun's rays are polluted by dung and by foul-smelling bodies, and do not remain pure amid such things.
If, however, according to the view of Celsus, the body of Jesus had been fashioned without generation, those who beheld the body would at once have believed that it had not been formed by generation; and yet an object, when seen, does not at the same time indicate the nature of that from which it has derived its origin. For example, suppose that there were some honey (placed before one) which had not been manufactured by bees, no one could tell from the taste or sight that it was not their workmanship, because the honey which comes from bees does not make known its origin by the senses, but experience alone can tell that it does not proceed from them.
In the same way, too, experience teaches that wine comes from the vine, for taste does not enable us to distinguish (the wine) which comes from the vine. In the same manner, therefore, the visible body does not make known the manner of its existence. And you will be induced to accept this view, by (regarding) the heavenly bodies, whose existence and splendour we perceive as we gaze at them; and yet, I presume, their appearance does not suggest to us whether they are created or uncreated; and accordingly different opinions have existed on these points.
And yet those who say that they are created are not agreed as to the manner of their creation, for their appearance does not suggest it, although the force of reason may have discovered that they are created, and how their creation was effected.
Source: Against Celsus (New Advent)