Both Calvinism and freewill theism affirm possible worlds. But what are possible worlds? What is the source of possible worlds? How are they constituted?
In Calvinism, I'd say a possible world is an alternate world history. God can and does imagine different scenarios. World histories with a plot, setting, characters. God's mind or imagination is the ultimate source or originator of these alternate scenarios.
It's less clear what possible worlds are in freewill theism. Let's approach it from a different angle. In freewill theism, what constitutes the future? Roughly speaking, the future is constituted by a combination of what God causes (directly or indirectly) and what creaturely free agents cause.
For instance, the physical world consists of many things that are not free agents. Natural processes. Rocks, trees, rivers, atoms, chemical reactions, lower animals. Presumable, a freewill theist will say God generally causes their existence or related events.
In addition are events caused by creaturely free agents, viz. humans, angels, demons. Let's use "free agent" as shorthand for human free agents. And let's define "free" in terms of libertarian freewill.
Finally, free agents interact with their physical environment, so some things that are not free agents in their own right might be caused by free agents. For instance, God caused the raw materials for a watch to exist, but metallurgy is a human activity. Glass production is a human activity. And it takes a watchmaker to design or build a watch. A watch is not a free agent, but the product of free agency. Selective breeding would be another example.
On a related note are the "counterfactuals of creaturely freedom". These are unexemplified choices by free agents. So we might say a possible world is, in part, a cluster of events effected by free choices. Volitions are events–mental events. And volitions can produce extramental events. A possible world is partly constituted by volitions and the effects of volitions.
On this view, a possible world is not so much where we'd do it, but what we'd do. Not so much that we'd do it in a possible world, but a possible world is what we'd do.
One complication is that, being merely possible, the agents don't actually exist. So what's the ontological status of possible worlds? What undergirds them?
We might suggest they are ontologically dependent on God's mind. God's mind is the repository of possible worlds.
God either knows what free agents would do or at least he knows all their possible courses of action. Those unexemplified agents and their choices subsist or inhere in God's mind.
But unlike Calvinism, they are still independent of God in the sense that choices have their ultimate source in the agent rather than God. Free choices are caused by the agent, not by God. God is the agent's originator while the agent is the originator of his own volitions.
If a free agent exists, it has that autonomous ability. That's an axiom of libertarian freedom.
This might explain why some possible worlds are infeasible. As Craig puts it:
The hypothesis is that God has done the very best He can, given the true counterfactuals of creaturely freedom which confront Him…God doesn’t create such a choice for Himself. The counterfactuals of creaturely freedom which confront Him are outside His control. He has to play with the hand He has been dealt.
Craig goes on to say:
But now you raise a quite different objection aimed specifically at (3). “Before God sticks Fred in second century Tibet wouldn't He have to ascertain that Fred would freely reject the Gospel in all circumstances, not just some of them?” Well, He wouldn’t have to, but that’s my hypothesis. Clearly, God could place a person anywhere He wants in human history, regardless of how that person might freely behave in different circumstances. But my suggestion is that God, being so merciful and not wanting anyone to be damned, so providentially orders the world that anyone who would embrace the Gospel if he were to hear it will not be placed in circumstances in which he fails to hear it and is lost. Only in the case of someone who would be saved through his response to general revelation would a person who would freely respond to special revelation, if he heard it, find himself in circumstances where he doesn’t hear it.
But there are problems with that explanation, even on its own terms:
i) If, by Craig's own admission, God can only play the hand he was dealt, what is Craig's justification for assuming the deck includes a feasible hand in which all the unreached people would refuse to believe the Gospel if given a chance? God lacks that magisterial control over the counterfactuals of creaturely freedom. That may not be in the cards. That option may not be available. At best, Craig can only hope it's true.
ii) It's also odd, on Craig's view, that historically, ethnic Jews and Europeans are far more likely to respond to general and special revelation than most Asians, East Indians, American Indians, sub-Saharan Africans, &c. How does Craig's theory account for the ethnic and geographical disparities?
iii) More fundamentally, is a possible world a place where agents make alternate choices? Or do free agents create their own circumstances? To the degree that circumstances are the result of free choices, how can God put a person anywhere he wants in human history, regardless of how that person might freely behave in different circumstances?
To a great extent, the situation in which we find ourselves was created by prior free agents. So how can God rearrange the order of cause and effect?
Libertarian freedom is typically defined as the ability to choose otherwise or do otherwise in the very same situation. You hold everything else in fixed position except for changing this one variable. Everything else is just the same leading up to this moment, this turning point. Everyone else does exactly the same thing. There's just this one discrete change–which produces other changes going forward.
There are possible worlds in which the past is identical up until that one alternate course of action. From a Calvinistic perspective, I can see how that would be. It's God imagining different outcomes. All this originates in God's mind. And most or maybe all but one possible world on exist as divine ideas.
But from a freewill theist perspective, why would there be gazillions of static world histories where you just have a single variation apiece? Consider a 4-way intersection. Each driver can turn left, right, or go straight. There's a possible world matching each potential choice by each driver. Where the other three drivers do exactly the same thing while the fourth driver does something different, or vice versa.
But why do the counterfactuals of creaturely freedom entail that a free agent would ring the changes on every logically possible alternative? Why would an agent want to? Just for the sake of sheer variation?
Put another way, if free choices in the past largely generate present circumstances, why would you have possible worlds in which the past is identical? Wouldn't the past be more fluid? Why is every other agent doing the same thing, and it's just one agent who does one thing differently?