Here's how I look at it. If God knows our futures, we have no free will. He knows exactly what we will do. Now, some people argue that he has no control over us individually; he just knows what choices we will make. However, knowing literally everything (and thus knowing what choices people will make) prohibits an individual's free will, because their fate is already written in stone before it has even been enacted. Even if God only knows what choices we will make, it stands to reason that those choices have already been made, thus how he knows we will make them. We could not change those decisions, because then God would not actually know our futures. The same applies for himself; God knowing his own future is a logical impossibility, because he would know all his choices before he made them, but would be powerless to alter those decisions. Thus, he is not omniscient. I know it's a very circular point to argue, but it just makes no logical sense.