Seeing, however, that Celsus quotes from an epistle of Plato another statement to the following effect, viz.: “If it appeared to me that these matters could be adequately explained to the multitude in writing and in oral address, what nobler pursuit in life could have been followed by me, than to commit to writing what was to prove of such advantage to human beings, and to lead the nature of all men onwards to the light?”— let us then consider this point briefly, viz., whether or not Plato were acquainted with any doctrines more profound than are contained in his writings, or more divine than those which he has left behind him, leaving it to each one to investigate the subject according to his ability, while we demonstrate that our prophets did know of greater things than any in the Scriptures, but which they did not commit to writing.
Ezekiel, e.g., received a roll, written within and without, in which were contained “lamentations,” and “songs,” and “denunciations;” but at the command of the Logos he swallowed the book, in order that its contents might not be written, and so made known to unworthy persons. John also is recorded to have seen and done a similar thing. Nay, Paul even heard “unspeakable words, which it is not lawful for a man to utter.” And it is related of Jesus, who was greater than all these, that He conversed with His disciples in private, and especially in their sacred retreats, concerning the Gospel of God; but the words which He uttered have not been preserved, because it appeared to the evangelists that they could not be adequately conveyed to the multitude in writing or in speech.
And if it were not tiresome to repeat the truth regarding these illustrious individuals, I would say that they saw better than Plato (by means of the intelligence which they received by the grace of God), what things were to be committed to writing, and how this was to be done, and what was by no means to be written to the multitude, and what was to be expressed in words, and what was not to be so conveyed. And once more, John, in teaching us the difference between what ought to be committed to writing and what not, declares that he heard seven thunders instructing him on certain matters, and forbidding him to commit their words to writing.
Source: Against Celsus (New Advent)