The problem of omniscience
Omniscience can be loosely defined as having the capacity to know everything. For most theists, God is omniscient, i.e., he knows the past, present and the future. He knows every detail of the Universe, he knows every part of our DNA, in short, he has infinite knowledge of everything that can ever be known to man.
Omniscience and freewill
If God knows that a man on a red shirt will crash a red car into a red fence in 30 minutes' time, then the outcome of that event is certain, if the outcome is certain, then the person is only following God's will, if the person is following God's will, then the person really has no will of his own. In other words, free will is an illusion.
Omniscience and omnipotence
Omnipotence means the ability to do anything. A terrorist detonates a bomb and kills 7 people, the police couldn't have stopped it because they had no knowledge of the event. If God had intervened to stop this terrorist, then he is truly not omniscient, he couldn't have changed an outcome that was already certain, if he did that, then the outcome wasn't really certain. Let me create another scenario, if a glass plate rolls off a table, either the glass plate will break or it won't break. If you stop it from hitting the floor, then there is nowhere you could have known whether the plate would have broken or not. For a god who knows the outcome, how can he stop it? If he stops it, he didn't really know the outcome.
Omniscience and God's word
The designer of a machine has a proper knowledge of how the machine works and understands its intricacies. The creator of the Universe should inspire a book that contains no ambiguities but with insights into our universe. When the Bible was written, Australia and America were not discovered, and people thought the earth was flat, how exactly has this so-called God's word guided us to understand the nature of the universe? How exactly has it led to scientific breakthroughs? There are so many portions of the Bible that cannot be practised or recreated in modern times. The Bible fails logically and scientifically as God's word. The Bible is not the work of an omniscient God.
Looking at omniscience itself
So theists believe that God knows everything, including his own future actions, he sees the past from the present and the present from the past. If God knows everything including his own future, then he doesn't have free will. A God who doesn't have free will couldn't have created the universe, because he needed to have thought about the universe before making it to come into existence. A god who doesn't have free will is not capable of making any decisions on his own. A god who doesn't have free will needs to follow the will of a higher god and so on and that would lead to infinite regress. If God is not omniscient, what makes him different from man? An omniscient god cannot and does not exist. A god who is omniscient is a product of man's invention.

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