In the next place, when Celsus says in express words, “If they would answer me, not as if I were asking for information, for I am acquainted with all their opinions, but because I take an equal interest in them all, it would be well. And if they will not, but will keep reiterating, as they generally do, 'Do not investigate,' etc., they must,” he continues, “explain to me at least of what nature these things are of which they speak, and whence they are derived,” etc. Now, with regard to his statement that he “is acquainted with all our doctrines,” we have to say that this is a boastful and daring assertion; for if he had read the prophets in particular, which are full of acknowledged difficulties, and of declarations that are obscure to the multitude, and if he had perused the parables of the Gospels, and the other writings of the law and of the Jewish history, and the utterances of the apostles, and had read them candidly, with a desire to enter into their meaning, he would not have expressed himself with such boldness, nor said that he “was acquainted with all their doctrines.”
Even we ourselves, who have devoted much study to these writings, would not say that “we were acquainted with everything,” for we have a regard for truth. Not one of us will assert, “I know all the doctrines of Epicurus,” or will be confident that he knows all those of Plato, in the knowledge of the fact that so many differences of opinion exist among the expositors of these systems. For who is so daring as to say that he knows all the opinions of the Stoics or of the Peripatetics?
Unless, indeed, it should be the case that he has heard this boast, “I know them all,” from some ignorant and senseless individuals, who do not perceive their own ignorance, and should thus imagine, from having had such persons as his teachers, that he was acquainted with them all. Such an one appears to me to act very much as a person would do who had visited Egypt (where the Egyptian savans, learned in their country's literature, are greatly given to philosophizing about those things which are regarded among them as divine, but where the vulgar, hearing certain myths, the reasons of which they do not understand, are greatly elated because of their fancied knowledge), and who should imagine that he is acquainted with the whole circle of Egyptian knowledge, after having been a disciple of the ignorant alone, and without having associated with any of the priests, or having learned the mysteries of the Egyptians from any other source.
And what I have said regarding the learned and ignorant among the Egyptians, I might have said also of the Persians; among whom there are mysteries, conducted on rational principles by the learned among them, but understood in a symbolic sense by the more superficial of the multitude. And the same remark applies to the Syrians, and Indians, and to all those who have a literature and a mythology.
Source: Against Celsus (New Advent)