So much so that some of his work in modal logic (necessity, possibility, counterfactuals, etc) is quite a good introduction to the subject. (see, for example, The Nature of Necessity: https://www.amazon.com/Nature-Necessity-Clarendon-Library-Ph...)
The problem of evil is an interesting one, in that you can take it two ways. On the one hand you might call it an internal problem to theism. Something like: "the Christian understanding of evil is incompatible with the Christian conception of God", and that's the argument Plantinga is primary addressing (that theism is not incoherent or internally inconsistent on this matter)
But you can also take it as an external problem - that theism doesn't do a good job of explaining the existence of evil. In my mind this is a stronger argument, but what sometimes gets lost in the discussion is that the problem doesn't go away if you discard theism. If evil really does exist what alternative worldview does a better job of explaining it?
That evil is just a social convention created by minds produced by random processes geared towards evolutionary fitness?
Maybe. But doesn't that just explain away the problem by stating evil doesn't really exist? (at least not the kind of evil suggested by Christians?) And if it doesn't really exist, how can you use it as an external argument against theism?
This conflates evil (a sentiment that we rational beings possess) with bad, one of the elements of metaethics. "Evil" is a nice label we use; "bad" is an ethical descriptor with any number of (non-religious) metaethical origins: consequentialism, deontology, virtue, &c.
The typical argument against against theism (or at least any sort of theism that claims that god is both omnibenevolent and omnipotent) is that there are bad things in the world: murder, theft, starvation, &c. When asked whether a good god would allow these things, we find ourselves dealing with (1) Euthyphro, and (2) incompatible properties (omnipotence and free will, omnibenevolence and divine command, &c).
The incompatibility of properties is precisely the issue Plantinga takes up. He offers a defense based on free-will. That there are possible worlds in which free people choose only to do good, but those worlds aren't actualizable, a sort-of transworld depravity, so any world God could create would have evil.
He doesn't present the defense as a theodicy, merely as a possible solution to the problem. And if there is any solution to the problem, then the properties aren't incompatible.
That is, it is logically possible for God to be omnipotent, omniscient and omnibenevolent and for evil to exist in the world.
Not sure. My personal experience with moral philosophers has been that most are (1) non-religious, and (2) committed to some form of moral realism. Personally, I'm not religious and think the morality exists as a necessary consequence of some vaguely Kantian sense of autonomy.
> That there are possible worlds in which free people choose only to do good, but those worlds aren't actualizable
I don't understand the distinction between "possible" and "actualizable" here -- in what sense is a state of affairs "possible" but not such that it could ever be actualized? Perhaps even more perniciously: how does the existence of a "transworld depravity" (which, presumably, is not itself god) not itself threaten omnipotence?
So if we imagine a free-will choice, say whether or not to mow the lawn today, it's possible for me to either mow the lawn or not, thus creating two possible worlds (the one where I do or the one where I don't).
God can't make me choose one way or the other, or rather, he can make me choose, but doing so makes the choice a non-libertarian-free one.
According to Molinism, God knows both the future (what I will actually do), but also all counterfactuals (what I would do in any situation).
He chose to create world we actually inhabit (the actual world), and there are many other worlds he could've created instead (actualizable worlds), but there are possible worlds that he could not have created, because many of the things that happen in those worlds are determined by the choices of free creatures.
So a subset of possible worlds are actualizable.
Plantinga is arguing that perhaps there are possible worlds with no evil, but none of them are actualizable.
Speculatively: God decides the trade-off is worth it and creates the universe anyway. Perhaps a universe in which God himself redeems free creatures from sin is better than one without free creatures at all.
Plantinga readily admits the speculation is just that. The important point is that the example illustrates a possible solution to the apparent paradox, so existence of evil and God's attributes aren't inconsistent. The actual solution may be something entirely different or even beyond our ability to comprehend.
As to the point about omnipotence: It's a good one, but I think in the end it boils down to the classic "can God create a rock so heavy he can't lift it" objection, and theologians will readily constrain omnipotence so as not to include the logically impossible, or that which is incongruent with his own nature. So by extension, that some worlds aren't actualizable doesn't impinge on omnipotence because it flows (logically) out of the concept of libertarian freedom.
Furthermore Molinism suggests a framework for thinking about how God can "ordain whatever comes to pass" if creatures have libertarian freedom.
It's also worth mentioning that the concept of libertarian freedom is itself widely debated. I would suspect that the majority of theologians are compatibilists when it comes to this topic, and don't believe that creatures have this sort of free will at all. (see the westminster confession of faith 3.2 for example) Some even argue that the concept itself may be philosophically incoherent.
Under a compatibilist view actualizable worlds and possible worlds are the same.