Well, I certainly know that I exist, because I have no way of percieving nothing since I have thought.
However, when it comes to superiority of knowledge, this is not necessarily superior to empirical knowledge. If you mean it is superior because it cannot be decieved, you have a point there, for empirical things can often be wrong until the true cause is isolated. However, take my knowledge of a chair and compare it to my knowledge that I exist. I know that I exist, and I know what a chair is. A chair is a chair, just as much as I am myself. I can describe to you the aspects of a chair, or the aspects of myself, I can tell you what a chair looks like, or what I look like, but I can't describe to you the form of a chair, just as I can't describe to you the form of myself. In both cases, I have the knowledge of [the chair or myself] through observation. I see a chair, which is a perception, and I percieve my own thoughts, another perception. I observe both, and if I know myself in any way better than I know a chair, it's probably because I have observed myself much longer. Math is another example. 2 + 2 = 4. This is absolute knowledge: the concepts involved will never change, nor will the equality (the symbols that represent them might, but not the actual equality), and we know this through observation. You can't observe 2 + 2 = 5. Someone somewhere had a group of 2 objects, and another group of 2 objects, put them together and had 4 objects. Or maybe s/he was just thinking about numbers, and realized that if you take two groups of two, that's the same quantity as 1 group of 4. We are not born with the knowledge of 2 + 2 = 4, we learn it as we grow.
Omniscience as we see it is to have such perfect information, that you can perfectly deduce what the effect will be for any given cause(which isn't actually a cause yet, but we'll call it that). This is an impossibility. It is impossible that perfect knowledge can come from a cause and effect thinking system since we've already proved that self-existence knowledge is perfect, and that it is a greater knowledge than empiricism. (if A > B and A = C, then B != C )
Let me get this straight. You're building a higherarchy of the perfection of knowledge of empirical observation against cogito ergo sum.
Secondly, self-existence knowledge was gained through empiricism. If I had no manner of perceiving/observing, I would not have observed that I think; therefore I would not be able to conclude that I am.
Additionally, the fact that self-existence knowledge is perfect does not imply that other kinds of knowledge are not perfect.
If A = true and A = A, well, that doesn't say anything about B.
Well, I think your reasoning is false, but the conclusion you came to is the same one I brought up in the post: Either god is not all-knowing or sarah has no free will.
By omnipresent, you mean god is at all possible points in space, but not necessarily all-knowing, and omniscient, you mean he has perfect knowledge of all, correct?
None.