After this the Jew makes another silly remark, saying, “How is it that, if Jesus pointed out beforehand both the traitor and the perjurer, they did not fear him as a God, and cease, the one from his intended treason, and the other from his perjury?” Here the learned Celsus did not see the contradiction in his statement: for if Jesus foreknew events as a God, then it was impossible for His foreknowledge to prove untrue; and therefore it was impossible for him who was known to Him as going to betray Him not to execute his purpose, nor for him who was rebuked as going to deny Him not to have been guilty of that crime.
For if it had been possible for the one to abstain from the act of betrayal, and the other from that of denial, as having been warned of the consequences of these actions beforehand, then His words were no longer true, who predicted that the one would betray Him and the other deny Him. For if He had foreknowledge of the traitor, He knew the wickedness in which the treason originated, and this wickedness was by no means taken away by the foreknowledge. And, again, if He had ascertained that one would deny Him, He made that prediction from seeing the weakness out of which that act of denial would arise, and yet this weakness was not to be taken away thus at once by the foreknowledge.
But whence he derived the statement, “that these persons betrayed and denied him without manifesting any concern about him,” I know not; for it was proved, with respect to the traitor, that it is false to say that he betrayed his master without an exhibition of anxiety regarding Him. And this was shown to be equally true of him who denied Him; for he went out, after the denial, and wept bitterly.
Source: Against Celsus (New Advent)