There is an a way of arguing from Calvinism to atheism. If the Bible is true, we have no libertarian free will (based on Calvinist arguments), but that means that God could have created us in such a way that everyone free does what is right, and everyone goes to heaven, but didn't. But a God who not only allowed sin, but also damnation, when God could just as easily have chosen their salvation is not a God worthy of worship. Hence, if the the God of the Bible exists, he is not worthy of worship (is not omnipotent, omniscient, and perfectly good), and hence there is no being in existence that satisfies this requirement. Therefore, atheism is true.
The closest I ever came to atheism was when I first encountered the biblical case for Calvinism. (I realize that someone will probably use this as the basis for an ad hominem argument against me. Remember, ad hominem is a fallacy.)
Reppert's attack on the God of Calvinism is an ad hominem attack. He's trying to discredit the God of Calvinism by impeaching his character. Therefore, Reppert's attack on Reformed theism is a fallacy.