For example, he is celebrated or vilified for his doctrine of predestination, despite the fact that he didn’t have such a “doctrine”—at least in terms of a unique view or emphasis. There’s nothing in Calvin’s teaching on predestination that isn’t also found in the great stream of Augustinian teaching. Thomas Aquinas taught double-predestination: that is, both unconditional election and reprobation. Luther imbibed this teaching from his abbot and mentor, who wrote a treatise defending “predestinating grace.” In fact, Calvin cautioned that, in his Bondage of the Will, Luther over-stated things in some places. [Emphasis Mine.]
Typical US democrat, in fact, other than lying about being a non-Catholic Christian while most democrat pols lie about being Catholic Christians. Then again, one does whatever it takes to get in office in the first place so if the local crowd is Protestant, so is the good lawyer who wants to control the city government.
Answer to headline: nothing.
Though there are some differences with Martin Luther, basically all Calvin or Luther ever did was to represent Saint Augustin to a world who had lost his incites.
It needed it desperately.
It needs it today in so many denominations.
Thomas Aquinas in his Summa Theologiae said "God does reprobate some.". But the question is how does he do it? God is not the author of sin, nor does He make anyone to sin. So how does He do it?
IMHO, it is simple. He lessens the restraint (boundaries) on their sin and lets them fall further into the corrupting nature of sin on the own.
A good example of this is Pharaoh's opposition to letting the Hebrews go to worship in the wilderness. After one of the ten plagues he would repent (a bit) and then turn around and oppose God yet once again. Romans says that God hardened Pharaoh's heart.
Again, how did God harden Pharaoh's heart without being the cause of his sin? My belief is that God moved some of the boundaries against evil and Pharaoh simply fell into it. Thus Pharaoh was responsible for his own disobedience and yet God's will was done.
How marvelous is God, far more subtle than man and whose ways are past finding out. But in some, we have a few clues.
Calvin’s Institutes is still the greatest book on systematic theology ever written.
The author is correct: Calvin’s errors are, in fact, endemic to the entire development of Western Christianity from about the point when Charlemagne’s court, able to read Latin but not Greek, raised Blessed Augustine to the status of the “Father among Fathers” and Westerners ceased to balance his views with those of Chrysostom and the Cappadocian Fathers.