I have a question about an argument for why the world must be completely deterministic if there is an omniscient God and wanted to know about the validity of the argument. If the premise is that God exists and is omniscient then God knows the future. If the future is already known by someone then it has to be already determined. Someone must have thought about this before already. Is there any literature on this? Is this argument valid?
No. While determinism could serve as a reason why a future is fixed and known to an outsider, determinism is not the only way this is possible philosophically.
Gods could not be bound by time an basically see all history including the future at once, as if everything had already happened.
Fatalism posits that some things are fated to happen, no matter what random things happen else, like a force that meddles with the universe to ensure certain future events.
Note that there may also be different forms of omniscient beings, knowing the future in all details does not have to be included.
Like the weather report being great about next week's weather, but not great about predicting next year's weather, a god could be great about predicting the next 100 years, but be less certain about the long term future, which would still feel like omniscient to puny humans.
It depends on the nature of "omniscience". IF God is inside time, then omniscience is not omniscience of the future, only of the past and present, and the nature of what might cause the future. As our physics is not deterministic, an omniscient God inside time does not imply determinism.
However, if God is eternal, and unchanging, and outside time, then whether God is omniscient or not, the future is determined. This is because a God outside time necessitates that the future is accessible to God, and this is only true for Block Time. If our universe has Block Time, then the future and past have identical characteristics, so our future is as determined as our past. https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time)
As a further observation, omniscience would require that God know the state of the universe. However, quantum mechanics behaves VERY differently for known vs unknown states, and our universe has multiple quantum interactions which could not happen if an omniscient being were aware of all quantum states at all times. Because quantum mechanics works, we can conclude there is no omniscient entity in our universe.
No, because of free will; free will "breaks" determinism. God is omniscient, but He has given beings free-will. That means that he knows everything (in a specific moment) regarding past, present and future if nothing changes. Whenever a change is made (by free will) all things are re-arranged. This solves the paradox.