I am convinced, indeed, that much better arguments could be adduced than any I have been able to bring forward, to show the falsehood of these allegations of Celsus, and to set forth the divine inspiration of the prophecies; but we have according to our ability, in our commentaries on Isaiah, Ezekiel, and some of the twelve minor prophets, explained literally and in detail what he calls “those fanatical and utterly unintelligible passages.” And if God give us grace in the time that He appoints for us, to advance in the knowledge of His word, we shall continue our investigation into the parts which remain, or into such at least as we are able to make plain.
And other persons of intelligence who wish to study Scripture may also find out its meaning for themselves; for although there are many places in which the meaning is not obvious, yet there are none where, as Celsus affirms, “there is no sense at all.” Neither is it true that “any fool or impostor can explain the passages so as to make them suit his own purposes.” For it belongs only to those who are wise in the truth of Christ (and to all them it does belong) to unfold the connection and meaning of even the obscure parts of prophecy, “comparing spiritual things with spiritual,” and interpreting each passage according to the usage of Scripture writers.
And Celsus is not to be believed when he says that he has heard such men prophesy; for no prophets bearing any resemblance to the ancient prophets have appeared in the time of Celsus. If there had been any, those who heard and admired them would have followed the example of the ancients, and have recorded the prophecies in writing. And it seems quite clear that Celsus is speaking falsely, when he says that “those prophets whom he had heard, on being pressed by him, confessed their true motives, and acknowledged that the ambiguous words they used really meant nothing.” He ought to have given the names of those whom he says he had heard, if he had any to give, so that those who were competent to judge might decide whether his allegations were true or false.
Source: Against Celsus (New Advent)