By its very definition, theism posits the existence of a deity which has in some way, shape, or form revealed itself to the universe. For such a revelation to be of any significance, there must be self-aware structures capable of free will. That is, these self-aware structures must be capable of producing some action in violation of causation, or for which there is no anticedent. If self-aware structures exist, but do not have free will, then the revelation is irrelevant.
Thus, the existence of any theistic deity must naturally and of necessity have the consequence of free-willed self-aware structures.
For free-will to exist, the self-aware structure must have some cognitive component which is non-tangible, and not subject to the principles of physics. Any tangible component is subject to physics, and thus violates the principle of non-causation. This principle of non-tangibility is, in psychology and philosophy, refered to as 'dualism'.
However, various studies have shown that without exception, every component of the mind and consciousness arises from the actions of the nervous system. These studies, in various ways, all involve selectively disabling portions of the brain or preventing communication between them -- split brain studies (where the corpus collosum has been cut or damaged), stroke victim studies (localized or regionalized brain death), and partial brain anasthesia. This establishes with high confidence, beyond reasonable doubt, that the mind is in fact monistic, or entirely tangible.
As stated, a free-willed mind must have a non-tangible component. Since the human mind has no non-tangible component, it is incapable of free will.
Since free will is a requirement of theism, theism is provably impossible, beyond any reasonable doubt.
This method of proof by modus tollens can be summarized:
[A Theistic Deity Exists] THEN [Free Will Exists].
NOT [Free Will Exists].
THEREFORE NOT [A Theistic Deity Exists].
Assign these as follows:
A = "A theistic deity exists."
B = "Free will exists."
Then the standard format is:
A⇒B.
¬B.
∴¬A.