In the next place, mixing up together various heresies, and not observing that some statements are the utterances of one heretical sect, and others of a different one, he brings forward the objections which we raised against Marcion. And, probably, having heard them from some paltry and ignorant individuals, he assails the very arguments which combat them, but not in a way that shows much intelligence. Quoting then our arguments against Marcion, and not observing that it is against Marcion that he is speaking, he asks: “Why does he send secretly, and destroy the works which he has created?
Why does he secretly employ force, and persuasion, and deceit? Why does he allure those who, as you assert, have been condemned or accused by him, and carry them away like a slave-dealer? Why does he teach them to steal away from their Lord? Why to flee from their father? Why does he claim them for himself against the father's will? Why does he profess to be the father of strange children?” To these questions he subjoins the following remark, as if by way of expressing his surprise: “Venerable, indeed, is the god who desires to be the father of those sinners who are condemned by another (god), and of the needy, and, as themselves say, of the very offscourings (of men), and who is unable to capture and punish his messenger, who escaped from him!”
After this, as if addressing us who acknowledge that this world is not the work of a different and strange god, he continues in the following strain: “If these are his works, how is it that God created evil? And how is it that he cannot persuade and admonish (men)? And how is it that he repents on account of the ingratitude and wickedness of men? He finds fault, moreover, with his own handwork, and hates, and threatens, and destroys his own offspring? Whither can he transport them out of this world, which he himself has made?”
Now it does not appear to me that by these remarks he makes clear what “evil” is; and although there have been among the Greeks many sects who differ as to the nature of good and evil, he hastily concludes, as if it were a consequence of our maintaining that this world also is a work of the universal God, that in our judgment God is the author of evil. Let it be, however, regarding evil as it may— whether created by God or not— it nevertheless follows only as a result when you compare the principal design. And I am greatly surprised if the inference regarding God's authorship of evil, which he thinks follows from our maintaining that this world also is the work of the universal God, does not follow too from his own statements.
For one might say to Celsus: “If these are His works, how is it that God created evil? And how is it that He cannot persuade and admonish men?” It is indeed the greatest error in reasoning to accuse those who are of different opinions of holding unsound doctrines, when the accuser himself is much more liable to the same charge with regard to his own.
Source: Against Celsus (New Advent)