if such a thing exists it means that free will does not, which means that you have no control over anything you do because its all been decided already, which means that this omniscient god made it that way, which means that you didnt have a choice whether to believe in him or not, which means that he made believers believe and disbelievers disbelieve, which means that the holy books (whichever one, take your pick) are riddled with lies and trying to make people convert/believe is fruitless
also raises the wonderful question of whether this omniscient god can create a rock so heavy that he cannot pick up