notesonthefoothills offers this proof of prophecy not being based on future foreknowledge:
What I mean is this. Eternalists would say that God knows all truths in a single logical moment. They will say that this includes God knowing his giving of resistible grace, the free movements of his creatures themselves, and his response to their movements. Thus God knows in a single Now what happens at t1, t2, t3, etc.. From this it follows that it also true that God knows that what happens, say, at t3 happens in part due to times that come BEFORE t3. In other words, God knows that each moment in time is what it is in part because of times that come before it. I am married in part because at some time in the past I proposed to my wife, I was raised in a certain part of the world, and I was born from my two parents, etc. Now from this comes an important point: it seems undeniable that this temporal, causal relationship is only ONE WAY. That is to say, I was not raised in a certain part of the world because I later married my wife; nor was I born because one day I would propose to her. That sort of EFFICIENT causal relationship applied such to temporal sequence is nonsensical. Grasping this point is essential to understanding my overall point here about prophecy.
How the point ties in to prophecy is this. It seems to me that in an eternal Now, God’s causal interaction with moments of time would likewise have to follow this same one-way causal relation. That is, how he interacts with t3 would be “because” of what occurs at t3 and also because of what occurs before t3. But it doesn’t seem possible that how he interacts with t3 would be “because” of what occurs AFTER t3. Here is why. If God uses what is after t3 to interact with t3 – say for instance that what occurs at t9 is his “because” for interacting with t3 in a particular way – then that would involve a causal loop insofar as the t9 that God is interacting with ALREADY HAS the preceding t’s as part of its causal history. So, I say that to say, it seems to me that God could not “see what happens” at t9 and use that to give a prophecy at t3. (I.e. God could not use knowledge gained at t9 to effect t3, because t9 already contains t1-8.) Unfortunately this is the most common response from Eternalists that I have read regarding how God makes prophecies in time.
Therefore it seems to me 2 things follow from this idea combined with the doctrine of God’s mode of existence: a) that God’s causal interactions with us, which involves true responsiveness and God doing things “because” of what we do in time, would uphold this logical relation among themselves. That is, God’s interaction at each stage would be “decided” by previous t stages, but not vice versa. His interaction at t3 would involve his interaction at t1 and t2, but not t4, t5, etc. This is because later t stages represent OPEN POSSIBILITIES with respect to God’s causal relation to us. And b) it seems NO prophecy which temporally precedes the event of which it prophesies about could come about with absolute certainty without God taking away free will. That is, if a prophecy occurs at t3 about t9, then God’s interaction at t3 has not yet (logically speaking) “taken into account” what freely happens at t9 (again, because t9 itself already contains t’s 1-8). God could of course impose his will so that the prophesied event came about necessarily; or he could give a conditional prophecy. But it seems to me the logic of eternity would preclude the idea of God using what occurs at later logico-temporal points to effect prior logico-temporal points, for that would involve a causal loop/regress.