Hello Brian,
Jacob pointed out the internal inconsistencies I referred to in my earlier post. Let me briefly explain the argument from the one and the many problem. You will see its immediate applicability to Islam. We observe reality both as a unity and a plurality. For instance, both Max and Lucy are dogs (unity); yet, they are two distinct dogs (plurality). For me to know Max, I at least have to have some conception of "dog-hood-ness" and at the same time know what makes Max different from all other dogs and objects for that matter. So, I should be able to grasp both what unifies Max with other dogs, and what distingushies Max from other dogs. The point being that knowledge requires unity and plurality.
The Greeks asked which of these two aspects of reality was ultimate. If plurality is ultimate, then reality consists at its ultimate base as a collection of things (a collection of things is even improper to speak of here) that are ultimately dissimilar (dualism +). Now, if unity is ultimate, then at the bottom we find one indivisible thing (monism). On either situation, nothing can be know because on the first hand, there is nothing that joins these dissimilar things, and on the second hand there is nothing to differentiate from the one.
Now, the Trinity provides and ontological basis to bring the two together. Neither unity not plurality is ulitmate over each other. Rather, they are two sides of the same coin - that coin being the ontological reality of the Christian God. Only on this type of conception where neither plurality or unity is ultimate can there be knowledge in principle. Since Islam does not provide such an ontological reality, then it is unable to account for this distinction that is a precondition for knowledge.
I would like to point out that any worldview that posits some ***inity (trinity, quadrinity, quintinity, etc...) would provide the same justification for this problem as the Trinity does. At least, I cannot think of a good reason why a god that is quadrinity fails to bring unity and plurality together.
Sincerely,
Brian
P.S. One thing that is interesting is that anything pressed hard enough and going back far enough like ethics, our universe, knowledge, etc...fails to be explained by simply nature. A supernatural ontological reality is necessary. But again, not any entity will do. This entity must be personal, must be able to bring the one and the many together, must be omni---, etc...When this point is made, only the Christian God fulfills all of these requirements. It is a good argument, but not necessarily an easy argument to make. To be a good presuppositionlist you must know your stuff cold. I disgaree with Bahnsen when he itimates that this is an easy apologetic. If you are unprepaired and come against an informed atheist (i.e., non-Christian whether thiest or not), you can find yourself in difficult waters.