A person in christ cannot be unsealed . The philosophy about omniscience ( including the heretical ' open theism) makes for interesting conversations, but I'd rather stick to the scriptures.
The Bible teaches Omniscience.
How does God prophecy things before they come to pass?
You either have to say foreknowledge or Divine causation, or both.
I say it depends. It could be foreknowledge or God could cause it. Depends on the prophecy.