In the next place, he objects to the statement, as if it were maintained by us, that “God will be able to do all things,” not seeing even here how these words are meant, and what “the all things” are which are included in it, and how it is said that God “will be able.” But on these matters it is not necessary now to speak; for although he might with a show of reason have opposed this proposition, he has not done so. Perhaps he did not understand the arguments which might be plausibly used against it, or if he did, he saw the answers that might be returned.
Now in our judgment God can do everything which it is possible for Him to do without ceasing to be God, and good, and wise. But Celsus asserts— not comprehending the meaning of the expression “God can do all things”— “that He will not desire to do anything wicked,” admitting that He has the power, but not the will, to commit evil. We, on the contrary, maintain that as that which by nature possesses the property of sweetening other things through its own inherent sweetness cannot produce bitterness contrary to its own peculiar nature, nor that whose nature it is to produce light through its being light can cause darkness; so neither is God able to commit wickedness, for the power of doing evil is contrary to His deity and its omnipotence.
Whereas if any one among existing things is able to commit wickedness from being inclined to wickedness by nature, it does so from not having in its nature the ability not to do evil.
Source: Against Celsus (New Advent)