It isn't just Calvinists, Arminians also believe God is omniscient. It is only the Open Theist and a few others (Mormons by example) that don't hold to Omni's.
So, first of all, I just want to say that it did not go unnoticed that you threw in the little ad hominem/guilt by association jab with the mention of Mormons. It's just another pathetic example of you throwing stones at a caricature of Open Theism.
Open Theists do not reject the Omni's per se, we simply modify them from what Classical theists, including both Calvinists and Arminians teach.
Open Theists believe that God is omnipotent, omniscient and omnipresent in the follow sense of these terms....
Omnipotent: God is the Creator of all things that exist besides Himself. As such, all power comes from Him. He has delegated real power and the real ability and authority to use that power to others. He retains both the ability and the absolute right to recall that power and authority at His sole discretion and is, therefore, not merely all powerful but is the sovereign power of existence.
This is very nearly identical to what most Christians believe! Mostly, in order to depart significantly from this understanding of omnipotence requires one to be "educated" away from it. Fortunately, most churches spend very little time discussing such things in much detail. Typically, the fact that God is omnipotent is merely declared from the pulpit, everyone yells "Amen!" from the pews and the preacher moves on to his next point.
Where the Open Theist departs from the Classical understanding of omnipotence is when theologians use it to introduce irrational concepts into their theology proper. We do not believe that God can go to a place that does not exist, for example. We do not wrestle with stupid conundrums like asking whether God can created an unmovable rock and then move it. It's a stupid question! NO! Of course God could not do any such stupid thing. There's a gigantic list of things that God cannot do, perhaps most important among them is the fact that God cannot make someone love Him. God cannot predestine that someone desires to have a genuine two way loving reciprocal relationship with Him (or with anyone else, for that matter).
The limitations Open Theists place on the other omni's are similar....
Omniscient: God knows everything that is knowable - that He desires to know.
Omnipresent: God is everywhere that exists at once - if He wants to be there.
Everyone seems to want to freak out over the qualifications we place on those two doctrines but it is just an application of omnipotence to those doctrines. All we are saying is that no one can force God to do something that He doesn't want to do. He isn't required to watch every single event that happens. God does not care about which photon of light is leaving off the back side of Polaris nor is He required to sit and watch while people commit gross sin nor is He omni-voyeuristic where He is unable to give people privacy if He so chooses to do so.
It takes more than Omniscience, however, to prove determinism.
Well, if within Omniscience you include exhaustive infallible foreknowledge of the future, as both the Calvinist and Arminians do then....
T = You answer the telephone tomorrow at 9 am
- Yesterday God infallibly believed T. [Supposition of infallible foreknowledge]
- If E occurred in the past, it is now-necessary that E occurred then. [Principle of the Necessity of the Past]
- It is now-necessary that yesterday God believed T. [1, 2]
- Necessarily, if yesterday God believed T, then T. [Definition of “infallibility”]
- If p is now-necessary, and necessarily (p → q), then q is now-necessary. [Transfer of Necessity Principle]
- So it is now-necessary that T. [3,4,5]
- If it is now-necessary that T, then you cannot do otherwise than answer the telephone tomorrow at 9 am. [Definition of “necessary”]
- Therefore, you cannot do otherwise than answer the telephone tomorrow at 9 am. [6, 7]
- If you cannot do otherwise when you do an act, you do not act freely. [Principle of Alternate Possibilities]
- Therefore, when you answer the telephone tomorrow at 9 am, you will not do it freely. [8, 9]
Source
In short, if you believe that we have a real ability to do or to do otherwise - to choose our actions - then the Classical understanding of Omniscience must be modified. Welcome to Open Theism!
If I get an almanac from the future, I've absolutely no ability to change outcomes. It is only if I purposefully interact to stop something, that the future could perhaps change (God can do that anytime). What it means, logically, is that there is no substance to the indictment: It doesn't matter what one knows, but rather if they act upon it. You'd say that a quarterback had no choice but to win the superbowl, but it doesn't add up. It is rather an enigma, rather than determinism, by proof of the almanac.
You are trying to have it both ways. You want to propose a situation where the future is known but not determined. That is a contradiction on its face. If the future is only known unless someone acts to change it then isn't it also known whether that action will be taken? Is the action that changes part of the future not itself part of the future?
The fact that such paradoxical nonsense is entirely unavoidable when discussing things like this is strong evidence, if not outright proof, that time does not exist. The past is what used to exist and the future does not yet exist. All that exists, exists now. Reality is perpetually in the present.
oo-su(r)