First of all, it is important to note that this and the Omniscient Mini-Marshmallows essay are completely unrelated. Mini-Marshmallows are not necessarily omniscient, thusly not forcing Free Will to be excluded.
Anyway, let me open with the conclusion: The concepts of Omniscience and Free Will are mutually exclusive. To understand this concept let us also view the basic premises:
#1 The commonly accepted definition of Omniscience
| Main Entry: om·ni·scient Pronunciation: -sh&nt Function: adjective Etymology: New Latin omniscient-, omnisciens, back-formation from Medieval Latin omniscientia Date: 1604 1 : having infinite awareness, understanding, and insight 2 : possessed of universal or complete knowledge - om·ni·scient·ly adverb |
As stated by the Merriam-Webster dictionary's web site.
#2 The commonly accepted definition of Free Will
| Main Entry: free will Function: noun Date: 13th century 1 : voluntary choice or decision <I do this of my own free will> 2 : freedom of humans to make choices that are not determined by prior causes or by divine intervention |
As stated by the Merriam-Webster dictionary's web site.
#3 Point #1 and #2 come to form the final premise: Omniscience must have infinite insight, including the outcome of all choices.
Now, if you consider that you cannot know the outcome of a free choice until the choice is made, that means that all choices must have been made prior to the existence of any omniscient being. However, Free Will mandates that all choices not be made until the need to choose arrives, which is what provides the "must be made before an omniscient being exists" because its coming into existence forces all decisions not yet made to be made at that point.
To ease the explanation, lets look at a basic example. Suppose thing O is omniscient. Now suppose person F is the individual making a decision. Now suppose that we are trying to come up with a situation where person O is omniscient, and will decide if free will was exhibited by person F. Person F has to choose between two choices, X and Y. F chooses X. Simple Yes? Now, lets look at it from object O's perspective and discuss F while forcing O to satisfy the definition of Omniscience. Now, O is omniscient, therefore, O knows F will choose X (Infinite Insight). Based on that F's choice is already made, he will pick F. Now, in order to satisfy O's omniscience, when F comes to the choice, F MUST pick X. F doesn't have a choice on this point. If F where to pick Y, O then did not have insight. But O has infinite insight, because O is omniscient, and thus F had to pick X. So did F freely choose, without prior cause or divide intervention? No. Because (Source of cause) O is omniscient, than F must pick X, and there is prior cause. O did not force F to pick X, it was the fact that O is omniscient that F had to pick X. Now if we hold that F has free will, and observe the effects on O, we can prove our mutual exclusion. Now F has a choice, F picks Y. Now O didn't know what F was going to pick, as F's decision was made without prior cause or intervention. Based on this O CAN NOT be omniscient, because O must have known (Infinite insight). Therefore, either F has free will, or O is omniscient, but not both.