It may help to think about this topic by eliminating the concept of time. What we want to ask is something like the following:
- Can an omniscient, omnipotent being 'turn the knobs' on a universe simulator containing free-willed creatures, such that none ever commits moral evil?
- If so, could he then just actualize such a universe?
It seems easy to think of a case where the answer to the first question is 'yes'. There is a term in probability called indefinite postponement, whereby a probabilistic process may never give a certain answer. While unlikely, you could flip a perfectly fair coin and never get heads. Couldn't it just be the case that the creatures never end up choosing wrongly? If the many-worlds interpretation is correct, there does indeed exist at least one world "without sin", to quote the Operative in Firefly.
But can we answer 'yes' to the second question? Here, we have to assume that the simulated universes are not real—that any evil in them is fictional and thus not bad—and that somehow, there is a way to reify such a universe. This smacks of "Last Thursdaysim", or the Omphalos hypothesis. Furthermore, there seems to be no guarantee that the now-reified universe will be immune to evil.
Wait a second, what about heaven? Couldn't we just "skip to the end", as one of my friends asks? Why not reify a simulated universe where no evil happened up and to the 'end', where 'end' can be described as a steady-state "everyone's singing praises to God"-type situation? There's a catch: it would be a moral evil for the creatures in heaven to think that they had a life before heaven, for that would be a lie: the reified thing was the end-state.
Plantinga answers 'no' to #1. More precisely, universes picked in #1 aren't guaranteed to be 'better' than universes where at least one evil action is committed. Perhaps in every world picked by #1, there are so many coincidences that normal science cannot be carried out.
In my view, God only runs into trouble if one of these is true:
- God can violate the laws of logic. (e.g. create a square circle)
- God can choose a different definition of 'good'.
If the answer to both of these questions is 'no', then it could be the case that all logically possible worlds with sinless [morally] free-willed creatures are less 'good', overall, than worlds with moral evil. Incidentally, I think this holds despite Plantinga's argument, but it is perhaps a reformulation: one can count 'good' on a per-creature basis, or on a per-possible-world basis.