Where do you get that notion from?
Theological Determinism
Theological determinism comes in two varieties. The first is based on the notion of foreknowledge: if God is an omniscient being, and if omniscience applies to the future (as well as to the past and present), then the future is known by God. But in that case, the future can only be what God knows it to be. No alternatives are possible. If God knows that it is going to rain tomorrow, then, regardless of what the weather forecast might be, it will definitely rain tomorrow. And if God knows that Jerry Falwell will decide to become an atheist sometime next week, then that is what inevitably must happen.
The second kind of theological determinism follows from the concept of divine preordination: if God is the ultimate cause behind everything, then He has preordained all that will ever occur, and once again there can be no deviation from the future's pre-set pattern. The preordination of the future is by definition a kind of determinism, so there is no arguing against it if one accepts the premise. (This is a type of causal determinism; the first kind of theological determinism is not.)
Theological determinism depends, of course, on whether or not God exists. Since there is no evidence for the existence of a supreme being, this kind of determinism is not logically compelling. And even if one accepts God's existence, it does not necessarily follow that God has preordained the future, or that He his omniscient, or that omniscience applies to the future. But for those with orthodox religious views things are not so simple. The notion of foreknowledge, at least, is essential to the orthodox concept of a supreme being. And yet the orthodox also wish to maintain the existence of free will, and thus reject determinism (though there are exceptions which are "orthodox" enough, e.g. Calvinism).
Now, some argue that knowledge of the future does not necessitate the future in any way: I may know that you are going to do x tomorrow, but that does not mean that you aren't freely choosing to do it. Whether or not this is right depends on how strongly we interpret what it means to have knowledge. If knowledge is supposed to imply certainty, then I cannot know that you are going to do x tomorrow unless I can somehow foretell the future. I may have very good reasons to believe that you will do x, and it may turn out that you will do x. But if it was the case that you might not have done x, then I did not really know.
One may of course use the term "knowledge" in a less strict sense in which the above would no longer apply. But when it comes to God's knowledge, as usually understood, there seems no doubt that it ought to be absolutely certain knowledge. God is after all supposed to be infallible.
Another common attempt to resolve the problem of foreknowledge is to claim that God exists outside of time. From this extra-temporal vantage point, God does not know the future beforehand. There is no "before" or "after" for God. Instead, He observes all of existence — past, present and future — as we observe the present. And as a result, His knowledge of the future is not foreknowledge, and so does not conflict with human freedom.
The "outside of time" argument is not easy to analyze because, so far as I can tell, no one knows what it really means to be outside of time. I believe the best way to tackle the argument is to consider how our situation is changed, if at all, on the supposition that God is in fact extra-temporal. God may be outside of time, but we're not. Now, for us the important thing is whether our future is something which is known. Wherever God is, if He knows our future, then from the vantage point of our present selves the future is in fact known. Our situation therefore has not changed in any way. The important thing is not how God possesses knowledge of the future, but that there is such knowledge. And if the future is in fact known, the conclusion that it is determined is unavoidable.
If the foregoing is not completely satisfactory (for as already mentioned the notion of extra-temporality is rather mysterious, so any discussion involving it is open to varying interpretations), there is another argument along the same lines. God, at least on most orthodox views, supposedly interacts with the world. But if there is such interaction with the world, then it must occur at particular points in time. And at those times, from our perspective, God certainly appears to have knowledge of our future.
Since I reject its premises, I do not accept theological determinism (the argument is valid, but not sound). Theological determinism is nevertheless important in that it reveals an inconsistency between the orthodox notions of foreknowledge and free will.
Theological Determinism