But if God knows what we're going to do, then doesn't that mean it's impossible for us to otherwise?
No. The people who say this are confusing "knowing" with "controlling". What God foreknows depends on what we will ultimately do. What we will ultimately do does not depend on what God foreknows.
Let's say that you need to choose between "A" and "B" tomorrow. God knows you will choose "A". Doesn't that mean it's impossible that you will choose "B"?
No. I could choose "B". In which case, God knows that I will choose "B". What God knows depends on what we will choose, not the other way around.
But I said that God knows you will choose "A"!
The only way that God knows I will choose "A" is if we're assuming that I will choose "A", that, for whatever reason, it's impossible that I will choose "B". Okay, so if it's impossible that I will choose "B", then, yes, I will not choose "B". But that's true whether an omniscient God exists or not. So your question here is *assuming* that I don't have free will. It's not something which is proven by the test, but an assumption on which the test is based.
Okay. But let's suppose that we put this to a test. You are about to choose between "A" and "B". God tells you, in advance, which one you will choose. Would you then be capable of choosing otherwise?
A-ha! If you did, then God would be wrong. In which case, He is not omniscient!
No, because God, being omniscient, would have known that I would choose otherwise. God knows that if He tells me I will choose "A", then I will choose "B" God knows that if He tells me I will choose "B", then I will choose "A". There is nothing here that God doesn't know.
Sure there is. God doesn't know what you will choose!
But there is no "what I will choose". What I ultimately choose is dependent upon what God does. He is capable of changing my actions based on what He decides to tell me. God also has free will, you see.
Let's get back to the test. What would happen?
I believe that God would simply refuse to take part in the test.
Let's say that God is honor-bound to go through with the test, to tell you what you will choose. And then let's say that you are honor-bound to choose the opposite of what He predicts. What would the result be?
One possible option is that God would cause me to mishear Him. Like, let's say He says "you will choose A". He would cause me to hear "you will choose B". Then, using my free will, and honor-bound to choose the opposite, I will choose A. Exactly as He predicted! I would still have free will, and God would still have predicted correctly.
But isn't that cheating?
Yes. No one says God can't cheat.
Okay, I'm saying it. What if we did this test, and God couldn't cheat. What would the results be?
Say that God predicts I will choose "A". So I choose "B". In that case, God predicts I will choose "B". So I choose "A". In other words, God would predict both "A" and "B", and I would choose both "A" and "B".
But you're only allowed to choose one, and God can only predict one.
Yes, and therefore this would cause a parallel dimension to open up, one in which God predicted "A" and one in which God predicted "B".
And He'd be wrong in both dimensions, right?
Nope. He'd be right in both dimensions, since right after He told me the two predictions, I would switch between the two dimensions. In other words, in one dimension, I'd hear God say "you will choose A", and then, just before I choose "B", I would switch over to the dimension in which God predicted "B". And His prediction would be right in that dimension. And in the other dimension, in which God predicted "B", I'd switch over to the dimension in which God predicted "A". I'd have free will in both dimensions, and God would have predicted correctly in both dimensions. Anyone in either dimension watching this test would see me choose exactly as God predicted.
So you're saying that God would create a parallel dimension in order to make the test work?
It would be the logical result of the test that you described, and would work exactly the same, I believe, if we used a time-traveler instead of God. A time-traveler who, after hearing my choice, went back in time to tell me my choice before I predicted it, would cause the exact same thing to happen.