For there are different appearances, as it were, of the Word, according as He shows Himself to each one of those who come to His doctrine; and this in a manner corresponding to the condition of him who is just becoming a disciple, or of him who has made a little progress, or of him who has advanced further, or of him who has already nearly attained to virtue, or who has even already attained it. And hence it is not the case, as Celsus and those like him would have it, that our God was transformed, and ascending the lofty mountain, showed that His real appearance was something different, and far more excellent than what those who remained below, and were unable to follow Him on high, beheld.
For those below did not possess eyes capable of seeing the transformation of the Word into His glorious and more divine condition. But with difficulty were they able to receive Him as He was; so that it might be said of Him by those who were unable to behold His more excellent nature: “We saw Him, and He had no form nor comeliness; but His form was mean, and inferior to that of the sons of men.” And let these remarks be an answer to the suppositions of Celsus, who does not understand the changes or transformations of Jesus, as related in the histories, nor His mortal and immortal nature.
Source: Against Celsus (New Advent)