A long time ago, someone objected to the existence of the Christian God by trying to point out a problem with the concept of omniscience. I’ve personally had this done more than once in a discussion with an atheist. The idea is that it is simply unacceptable that God could know exactly what one was going to do in the future.
For instance, the skeptic invites us to imagine an omniscient being. This being is, by definition, correct in whatever he says concerning his knowledge. He hands you a sheet of paper and a pen. Let us also assume you agree to write a number of any length (single digit on up) on the paper. Assume he will tell you the truth. He then claims, “I am going to tell you the number you are going to write next on the sheet of paper.” The claim by the skeptic is that whatever number the omniscient being mentions, you may simply add one to it and write that down instead. So if the being says, “
41,” you may write “ 42.” Since it seems well within your power to do this, the reasoning goes such a being cannot exist. This is inadequate for a number of reasons, however.
First, this argument, at best, is only against the traditional Christian conception of God. That is to say, even if everything said is granted as true, the conclusion, “Therefore, the God of Christianity does not exist” is unwarranted. Perhaps then the Open Theists are correct and God simply does not know the future.
Second, the conclusion that an omniscient being cannot exist does not follow even from the premises given. In the story, the thing that supposedly makes omniscience impossible is that the finite knower is given future knowledge of events that are directly within his control. What is to say that an omniscient being would (or should) not simply refrain from telling the finite knower this information? The problem evaporates when the finite knower is ignorant concerning the future event.
Third, implicit in this argument is that a denial of the conclusion indicates that omniscience must control future free actions. One of the parameters is that writing down one number higher is in fact within one’s control. Of course, theological determinists may not agree to this, and simply respond such an omniscient knower may in fact be omnipotent to force the future act, and the free act cannot be performed. That is to say, omniscience does not guarantee freedom. Or it could be objected that since omniscience cannot be shown from this example alone (on pain of question-begging) to be determinative of future events, it can be denied that such a being’s knowledge must determine the outcome. If that is the case, one must also consider that the omniscient being, by definition, knew what the finite knower would do in those circumstances, and could have planned accordingly. (This just ties in with the second point; if an omniscient being committed to telling the truth 100% of the time knew that by such an exercise he would cause the finite knower to write another number down, he would refrain from informing the finite knower in the first place.)
Finally, one must notice the most glaringly obvious piece of information: the parameter is that the being is omniscient. Since the thought experiment has not given us any outside argument or premise to consider, it simply assumes that the omniscient being is wrong! It therefore assumes what it seeks to prove, and hence is question-begging.
Now one may object that it still seems as though the finite knower is not free to write down another number; but it seems as though this need not be the case. As William Lane Craig points out, we may suppose that the omniscient being says you’ll write down a 6, and you intend to write a 7, but you think better of it (perhaps because you are a Christian and wish to ensure God is correct [but under the parameters God would know that beforehand too!]); or you intend to write a 7, but you slip in such a way as to make the unmistakable shape of the 6. Your free intentions and choices are still free, but the objection only holds if we already assume omniscience and free will cannot co-exist.
 For those of us who think this answer is generally unacceptable (such as myself), there are alternatives.
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