Firstly, Molinism is an attempt to resolve tension between God's sovereignty and man's allegedly free [extrinsically, antecedently uncaused] will. Essentially, it is the position that, because God is omniscient, He knows what His creatures will "freely" choose in any hypothetical circumstance. Molinists call these hypotheticals "possible worlds." From these possible worlds, God has "sovereignly" chosen to effect one. The Molinist's contention is that God's choice to effect this possible world was unconditional - so God was in control ("sovereign") - yet, because this possible world is one in which we have "freely" chosen (given the circumstances), there is no mutual exclusivity between God's sovereignty and man's allegedly free will.
Secondly, Molinism is an attempt to resolve tension between God's omniscience and man's allegedly free [extrinsically, antecedently uncaused] will. Matthew 11 is a passage in which Jesus reveals that He knows what would have happened in a different possible world. Molinists attempt to reconcile that fact with God's knowledge by distinguishing between types of God's knowledge:
1. Necessary knowledge - knowledge of what must be (e.g. self-knowledge)
2. Middle knowledge - knowledge of what would be (e.g. possible worlds)
3. Free knowledge - knowledge of what is (e.g. the actual world)
Middle knowledge includes knowledge of what men freely choose and is the type of knowledge about which I spoke in the first paragraph, the one by which God was able to execute His sovereignty. This is thought to be sufficient explanation as to how God's omniscience is compatible with man's allegedly free will.
The primary problem with Molinism pertains to how God's omniscience can harmonize with alleged creaturely freedom.
The difference between God's alleged "types" of knowledge is logical. Free knowledge, for example, is logically dependent on middle knowledge, for God could not know what "is" without knowing what "could be." The problem is that Molinists provide no explanation as to how God possesses knowledge of what men would freely choose; that is, how does God know what "would be"? They beg the very question which is in need of answering, i.e. how God knows what men will freely choose. Steve Hays summarized this dilemma as follows:
What is a possible world? What is a possible agent?
Here I’m puzzled by the position of Craig, Plantinga, et al. They treat possible persons as a given. Given possible persons, with determinate character
traits, God chooses which world to instantiate in light of what possible agents would do in different possible worlds. He chooses the possible world which achieves his objective.
But it’s unclear to me how Craig, Plantinga, et al. account for the given. How do possible persons subsist, with determinate character traits, such that God’s choice is responsive the free choices?
To me, the only logical way to embed this notion, consistent with libertarianism, is to go the route of Richard Creel. There’s a platonic plenum which is populated by possible agents in possible worlds. This exists independently of the divine nature or will. It’s like a mail order catalogue from which God can make his selection.
But, of course, the notion of a coeternal, self-subsistent plenum, alongside God, is profoundly heretical. It’s also metaphysically bizarre. What is the plenum? Is the plenum a mind-like entity?
Molinists essentially treat middle knowledge as a brute fact. However, how God knows what men would "freely" choose in a hypothetical situation is not dissimilar to the question regarding how God knew what I have actually chosen with alleged free will. If God doesn't cause our choice (directly or indirectly), God's knowledge (whether of the actual or a possible world) must itself be predicated on our [temporal] will, in which case God is not eternally omniscient.
In fact, the only thing Molinists present which remotely resembles an argument is that middle knowledge is the only position compatible with God's knowledge of counter-factuals. But consistent Calvinism provides a more cogent answer to the issue: God ultimately causes all things, so He knows counter-factuals because He knows what would have happened had He ultimately caused events other than as He actually has.