It may be clearer to restrict the concern to Plantinga’s “naturalism” which implies a belief that there is nothing “God-like”, that is, there are no agents and everything is the result of event causation. See Plantinga’s Evolutionary Argument Against Naturalism (EAAN) in Where the Conflict Really Lies.
The problem of evil may challenge not just a certain group of theists who believe in an omnipotent, omniscient and omnibenevolent God, but it also challenges this naturalism.
The problem of evil has four premises assuming the concurrent existence of both a particular type of God and evil:
- God is omniscient (all-knowing)
- God is omnipotent (all-powerful)
- God is omnibenevolent (morally perfect)
- There is evil in the world
The conclusion is that these four premises lead to a logical contradiction. If one agrees with that, one would have to reject some of the premises. Perhaps an omniscient, omnipotent and omnibenevolent God does not exist? This removes God, but it keeps evil. One might also claim there is no evil and keep God.
Plantinga’s free will defense defuses this argument. He shows how one can keep all four premises by showing that they do not result in a contradiction given the existence of agents with free will.
A naturalist, as described in Plantinga’s EAAN, should not accept any of the four premises as true. In particular the naturalist should not accept the fourth premise about the existence of evil any more than the first three premises about the existence of God. Evil requires an agent to cause the evil, but naturalists, as Plantinga describes them, do not admit anything “God-like”: They do not admit the existence of any agents that might bring about evil.
Evil might be watered down to “natural evil” so that evil is reduced to some event causation that some might not be happy about but conceivably others might. Regardless how one feels, since that is nothing more than determined neural brain processes, there is nothing one can do about it since this “one” who wants to do something about it is not an agent.
This naturalism would be challenged if one could show the existence of an agent, that is, if it could be shown that at least one of the four premises of the problem of evil argument are true. That agent might be God perhaps like the one described in the first three premises. That agent may also be the one causing any evil that might exist in the world.
It is easy to dismiss God who can be viewed as transcending empirical evidence, but perhaps it is not so easy to dismiss evil that some find more evident. One would have to convert anything evil into an event causation that neutralizes it and claim that those who see evil in the world are deluded just as they are deluded about their free will.
The existence of injustice or evil is more specifically a challenge to views such as naturalism, which as Plantinga describes it is an extreme form of atheism, that do not admit the existence of any agent to cause injustice.