In Luther’s case, the prospect of judgment by the terrifying God of nominalism and voluntarism – an omnipotent and capricious will, ungoverned by any rational principle – was cause for despair. Since reason is incapable of fathoming this God and good works incapable of appeasing Him, faith alone could be Luther’s refuge.I had heard that nominalism and voluntarism were ideas put forward by William that sowed the seeds for the reformation. But how did one produce the other? Ockham seems like a reaction to the power of St Thomas of Aquinas. When God gives revelation we either bow down to it or fight it. William wanted to fight what St Thomas had put forward by faith and reason. So he attacked one of the central building blocks of Scholasticism. He denied the existence of essences. That had the effect of reducing the power of reason. It meant all that could really be known about faith and morals was from revelation. God's will was not predictable from God's essence and reason. God's will simply was.
So when Luther desires assurance of salvation the Catholic answer to that is our assurance flows from God's grace working in our lives. When we are made holy in ways we know we could not achieve ourselves we can be sure that God is real and that He is saving us. But what if we look at our life and simply don't know if we have received supernatural grace? This is possible if we lose our rational basis for determining good and evil. If we cannot make any sense of what God does then we can only have assurance of salvation based on what we do? So he latched onto those passages which talked about salvation by faith. Then he committed the error of every heretic. He used his pet doctrine to knock down other doctrines and change the faith.
I am quite interested in the root causes of the reformation. Why did someone become totally convinced of an error and why were they able to convince so many others? Often the answer to these questions lies in philosophy. Bad philosophy almost always leads to bad theology. Where did the bad philosophy come from? I see it as a running away from the bright light of Aquinas.
Certainly there were more factors. There were immoral popes and bishops around that time. There was the invention of the printing press. The development of universities. Lots of things that made the 16th century Europe ripe for the spread of heresy. But why this heresy? How did Luther get so far from Catholicism so fast and with such moral certainty? Once your have rejected the philosophical underpinnings of a system then nothing built on it has any strength. Large swaths of settled doctrine can be discarded in an instant.
The scary part is people are often unaware of their underlying philosophy. We can't examine it against scripture because our reading of scripture is colored by our philosophy. The only way we can tell is when we start coming to conclusions that contradict dogmatic teaching. Then we know we have to rethink things. But that is hard to do. It can mean changing your mindset quite radically. It is so much easier to just pretend those problems are not there.