What God can and cannot do
Then he raises an objection, as though we said it: “God will be able to do all things.” He does not even see how this is said, what “all things” are included in it, or how God is able. There is no need to speak of these matters now; for he himself did not take a stand against the saying, though he could have done so plausibly. Perhaps he did not follow the plausible case that could be made against it, or he followed it but also saw the reply to that argument. In our view God can do all things that he can do without departing from being God, from being good and from being wise.
But Celsus, as one who has not understood how God is said to be able to do all things, says: “He will not wish anything unjust,” granting that he can do even what is unjust, but does not wish to. We say that, just as what sweetens by nature, because it is sweet, cannot make things bitter against the one cause by which alone it acts, and what gives light by nature, because it is light, cannot make things dark, so neither can God do injustice; for the power to do injustice is contrary to his deity and to all the power that accords with it. And if any being can do injustice because its nature is fitted for injustice as well, it can do injustice since it does not have in its nature the inability ever to do injustice.
Greek
Εἶτα ἀνθυποφέρει ὡσπερεὶ λεγόμενον ὑφ᾿ ἡμῶν τὸ δυνήσεται πάντα ὁ θεὸς, οὐδὲ τοῦθ᾿ ὁρῶν πῶς λέλεκται, καὶ τίνα πάντα ἐν τούτῳ παραλαμβάνονται, καὶ πῶς δύναται. περὶ ων οὐκ ἀναγκαῖον νῦν λέγειν, οὐδὲ γὰρ αὐτὸς. καίτοι γε δυνάμενος πρὸς αὐτὸ στῆναι πιθανῶς. ἔστη· τάχα μηδὲ παρακολουθῶν τῇ λεχθησομένῃ ἂν κατὰ τούτου πιθανότητι, ἢ παρακολουθῶν μὲν θεωρῶν δὲ καὶ τὴν πρὸς τὸ λεγόμενον ἀπάντησιν. δύναται δὲ καθ᾿ ἡμᾶς πάντα ὁ θεὸς, ἅπερ δυνάμενος τοῦ θεὸς εἶναι καὶ τοῦ ἀγαθὸς εἶναι καὶ σοφὸς εἶναι οὐκ ἐξίσταται. ὁ δὲ Κέλσος φησὶν ὡς μὴ νοήσας, πῶς λέγεται ὁ θεὸς πάντα δύνασθαι, ὅτι οὐκ ἐθελήσει οὐδὲν ἄδικον, διδοὺς ὅτι δύναται μὲν καὶ τὸ ἄδικον, οὐ θέλει δέ. ἡμεῖς δέ φαμεν ὅτι. ὥσπερ οὐ δύναται τὸ πεφυκὸς γλυκαίνειν τῷ γλυκὺ τυγχάνειν πικράζειν παρὰ τὴν αὐτοῦ μόνην αἰτίαν. οὐδὲ τὸ πεφυκὸς φωτίζειν τῷ εἶναι φῶς σκοτίζειν, οὕτως οὐδ᾿ ὁ θεὸς δύναται ἀδικεῖν· ἐναντίον γάρ ἐστιν αὐτοῦ τῇ θειότητι καὶ τῇ κατ᾿ αὐτὴν πάσῃ δυνάμει ἡ τοῦ ἀδικεῖν δύναμις. εἰ δέ τι τῶν ὄντων δύναται ἀδικεῖν τῷ καὶ πρὸς τὸ ἀδικεῖν πεφυκέναι, δύναται ἀδικεῖν οὐκ ἔχον ἐν τῇ φύσει τὸ μηδαμῶς δύνασθαι ἀδικεῖν.