Here’s an argument I stumbled upon on Richard M. Gale’s website when browsing his course outline for the philosophy of religion class he used to teach.
1. God is essentially omnibenevolent.
2. God is essentially free.
3. God cannot do what is morally wrong. From 1
4. God is not free. From 3
5. God is free and God is not free. From 2 and 4
This argument seems pretty easy to see through from my perspective, as it seems that 2 and 4 are not incompatible, since 2 says that God is essentially free, and 4 should say that there are some things which God is not free with respect to. One could make an analogous argument against God’s omnipotence by asking whether God is able to cause himself to cease to exist – what is proposed is a confusion of language, and therefore non-sense. God is that than which nothing greater could be conceived, which entails necessary existence, and thus to propose that he (that which exists such that it cannot not exist) could cease to exist is simply not an intelligible proposition (since the subject and predicate cannot be intelligibly related). Similarly it is not logically possible for God to be free with respect to doing something contrary to his nature, which is precisely what sinning would entail. Therefore God is not free to sin, but there are some things with respect to which God is indeed free, such as with respect to whether he should create a world or not. God does not necessarily create a world, but instead freely chooses to do so in harmony with his loving self-sacrificial Trinitarian nature (note, though, that to not create would also have been in harmony with that nature – but sinning would not have been in harmony with that nature). Other things with respect to which God is free would include choosing to actualize the Christian story by becoming incarnate, or indeed answering prayer, or deciding how precisely to best answer prayer, etc.