So, I'm kind of agnostic between Thomistic and Molinist views of predestination at the moment (though sympathetic to the Thomistic view). I'm worried about the Thomistic view since if God's will is efficacious then he should be able to bring about the salvation of all, and so the only explanation for why not all are saved is that God does not will it, which is contrary to Scripture. Aquinas replies by distinguishing between God's consequent and antecedent will. So God in general ('antecedently') wills that all men be saved, but taking all things into consideration ('consequently') wills that only some are saved; just as a judge wills that all men should live but taking into consideration particular cases wills that some should die. However, the difference between the two cases is that God's will can never be thwarted or perverted.
The problem with the Molinist view in my book is that I don't think it fits well with God's providence/omnipotence. The Molinist will reply that there's no problem, since it's not contrary to God's omnipotence if God can't bring about something that's impossible, and God's causing someone to freely choose him is impossible, since a free action can't be determined. But here I see no reason why God cannot bring about either of two contraries of a person's choice (i.e. either action A or not-A) and the action still be free--for God and creatures are working on different causal plains so to speak; God creates 'ex nihilo' by giving the whole of existence to particular states of affairs, whereas creatures cause in the sense of interacting with each other. It is only causing in the latter sense of interacting with by coercing or forcing that is incompatible with free will, and this is not the sense in which God is a cause.
To elaborate on the Thomistic point about how God can cause free actions we must distinguish different senses of the word 'cause.' First there is the sense of 'cause' in terms of interacting. Let's call this 'cause1'. So, for instance, this is what it means when I push you and 'cause' you to fall down, or when neurons cause arms to move, or if Cartesianism is true what happens when the soul causes the body to move. God does not cause in this sense; God does not cause1.
There is another sense of cause which means to create ex nihilo at evey moment, to 'sustain' or give being to things (although I have some reservations about the word 'sustain' since I think it can be misleading, since God isn't in time). We can call this cause2. God causes in this sense by being the cause of all being other than himself. He gives existence or 'esse' to everything. This is a very central doctrine to Thomism. God is the only person who causes in this sense; God is the only person who can cause2. God can cause2 anything that is possible. And on libertarian free will either of two contraries--i.e. either of action A or ~A--is possible, so God can cause2 either. Though I would agree he can't cause1 them, since that would entail determinism.
Now, maybe one can argue that even in the sense of cause2, this counts as a form of determinism which limits moral responsibility, and hence we should revert to the Molinist response. In Part II I'll respond to this point.