What do you believe?
Note that the proof does not say that professed unbelievers do not prove things. The argument is that you must borrow from the Christian worldview, and a God who makes universal, immaterial, unchanging laws possible in order to prove anything.
This type of logical proof deals with "transcendentals" or "necessary starting points," and the proof is called a "transcendental proof." Any contrary view to the God of Christianity being the necessary starting point for rationality is reduced to absurdity. You have to assume God in order to argue against Him.