Image by Deeo Elaclaire at http://rubiconthemovie.com/all-the-worlds-a-stage/parallel_worlds_by_deeo_elaclaire/
Ever since my conversation with Mark Hall on The Ontological Argument (in the comment section of that article), I've been doing my best to figure out the simplest way possible to explain why a world with universal salvation and 100% moral goodness is an infeasible world for God to create.
I think one way to think of it is this: What if a person (we'll call him Sam) *would not* under any circumstance choose a particular thing (we'll call it A). In that case, a world where Sam chooses A freely is infeasible for God to actualize because God knows Sam would never choose A in any circumstance. Or, what if Sam would choose A in a certain circumstance, but he would not choose A in circumstance S. In that case, a world where Sam chooses A freely in circumstance S is infeasible for God to create. Although a world where Sam chooses A in circumstance T is quite feasible.
Even though it’s logically possible for there to be a world where Sam chooses A freely, it may be infeasible because that is not the direction Sam exercises his will.
In the case of salvation, and in the case where far more free agents are interacting with one another, I find it very likely that many of the circumstances God knows will be adequate to extract a free response on the part of the person are non-compossible. They can’t all be cobbled together in a single world. Trying to give adequate illustrations to show how the salvic-circumstances cancel each other out is practically impossible, and all illustrations I’ve attempted to give were severely limited in scope. It’s humanly impossible for me to give illustrations of possible worlds involving as many free agents as this one has (so far) and all the consequences their free choices *would* bring *if* a certain number of factors are in place. Any illustration as to why God cannot use His middle knowledge to actualize a world of universal salvation will be narrow in it’s scope. I cannot give an illustration of a 50,000 year human history with billions of free agents and how their actions affect the outcomes of other actions in a given possible world. That’s why I’m doing my best here to give the simplest explanation possible that will get the point across.
Although a world where Sam chooses A in circumstance T is quite feasible. A world where Bob chooses A in the same world where Sam chooses action A in circumstance T is infeasible. For if circumstance T comes about, Sam will choose A, but Bob will refrain from choosing A. If God actualizes circumstance S, Bob will choose A, but Sam will choose B. But what if God wants Bob to choose A and for Sam to choose A. If Circumstance T negates Bob choosing A and brings about Sam choosing A, then a world where both Bob and Sam make the same choice in circumstance T is infeasible for God even though it’s logically possible. Of course, if you take out the free will factor and God steps in and *makes* either Bob or Sam choose what He wants, then the proposition “Bob and Sam both chose A in circumstance T” can come true. But, that’s not what God wants. God wants Bob and Sam to both choose A freely, uncoerced. That’s what infeasible for God to do. Even infinite power cannot make Sam and Bob choose A in circumstance T IF Bob and/or Sam would choose to exercise their wills differently in placed in T. Only if Bob and Sam would freely choose A in circumstance T would a possible world where Bob and Sam both freely choose A in circumstance T be feasible for God to create.Whether or not its feasible for God to create the world where both Bob and Sam both choose A in circumstance T depends on how Bob and Sam choose in circumstance T. Whether they choose A AT ALL in ANY circumstance is up to them. They are free agents after all. And if they simply refuse to do what God wants, God cannot make them do it without abolishing their free will. It's as logically impossible to force someone to do something freely as it is for God to create a rock so heavy that He cannot lift it.
I hope this clears things up.