The foreknowledge argument is supposed to establish the truth of determinism. I think it works in the favor of determinism (of the divine kind not the naturalistic, causal kind). Whether or not one is convinced by the conclusion of the argument is dependent, at least in part, on whether or not one can come up a valid form of the argument. In this post I intend to provide one (but not invent or originate one; only to repeat one from my elders and betters).
Take any claim you like–say, for the sake of illustration, that I will be involved in a traffic accident tomorrow. Now, we may not know yet whether or not this is true, but we know that either I will be involved in an accident or I won’t. Suppose the first of these. Then, as a matter of fact, I will be involved in a traffic accident. And if it is true to say that I will be involved in a traffic accident and Then it cannot fail to be the case that I will be involved. That is, it must be the case that I will be involved. Suppose, on the other hand, that I will not, as a matter of fact, be involved in a traffic accident tomorrow. Then it is true to say that I will not be involved in an accident; and if this is so, it cannot fail to be the case that I won’t be in an accident. That is, it must be the case that I am not involved in an accident. Whichever of these two does happen, then, it must happen. This is fatalism.
Priest suggests that the argument trades on an ambiguity. The truth that is established in the argument is that if it is true to say something, then whatever it is you have said is the case. However, the argument fails to establish the necessity of the following claim: if it was in fact the case that it was true in the past that Graham would suffer in a car accident then there is no possibility that he wouldn’t.
“So: (i) necessarily, yesterday God infallibly believes that you will do X today, and since (ii) God’s infallible belief that you will do X today entails that you will do X today, then (iii) necessarily you will do X today” (Paul Manata, Free Will, Moral Responsibility, and Reformed Theology: A Contemporary Introduction).
God knew at t1 that A will do X at t2. If A does not do X at t2 then God does not know the future. Or God knows timelessly that A will do X at t2. Either way, though not logically entailed, the outcome is necessitated (accidentally). It might have been true at t1 that A will do X at t2, but that is not what the argument suggests. What the argument suggests is something happened at t1. If something happens at t1, then there is noting one can do to change it at t2. Something did happen – a state of affairs obtained at t1 in the form of God believing that at t2 A will do X. Propositions are not the kinds of things that obtain or take place; they are the kinds of things that are true or false. Therefore, the foreknowledge argument stands (Zagzebski, The Dilemma of Freedom and Foreknowledge, 28-31)