Nominalism Didn't Make Him Do It
Theologically, he followed Louis Bouyer in seeing the Reformation as a confused and damaging attempt to escape the serious errors of late medieval scholastic theology, especially nominalism, a subject about which Chris Jones once made a post here.
But I find this argument unconvincing. First of all, nominalism appears to be far more complex and multivocal theologically than Bouyer allowed, having theological positions "all over the block." Alistair McGrath has made this point at length (I summarize it here).
Further, he argues that because of nominalism Evangelical theology cannot assign any meaning to the idea of sanctification, and hence flatly denies its possibility, something which I was not aware Evangelical theology asserts. Evangelical theology denies that one can find assurance in sanctification, but it does not deny that sanctification happens; indeed sanctification inevitably takes place in the believer. Mascall offers no citations of Luther -- or of the nominalists, most of whom were completely comfortable with the Catholic affirmations that Luther denied -- and without that he's hard put to convince me that he's really hit the key points.
I'm also a little wary since he obviously gets wrong the root of Luther's objection to the "sacrifice of the Mass" idea; it's not fundamentally about whether Christ must die numerous times, so much as it is about synergy of sinful man with Christ in propitiating God. A Catholic might argue that Luther's wrong, but that's the major argument that has to be met.
Nor can I follow Mascall when he follows Bouyer in saying that Erasmus was a good Catholic Christian who was just forced by his nominalism (shared with Luther) to sound like a Pelagian for whom the grace of Christ hardly matters. I've read Erasmus against Luther -- several times -- and I have concluded he WAS a Pelagian for whom the grace of Christ hardly matters. Nominalism did not force Erasmus to write "St. Socrates, pray for us."
And finally of course there is the missing Bible. I would like to suggest that one's interpretation of a relatively small number of passages will determine one's theology.
Romans 7:14-25: Is this about Paul as an unbeliever, as a new convert, or as a mature believer? Jerome said the first, Augustine the second. The second leads straight to simul iustus et peccator.
The Law Paul says cannot give righteousness in Romans and Galatians: is it only the ceremonial law as Jerome says, or does this inability extent to the moral law, and the Ten Commandments, as Luther said? If the first, justification by faith apart from the Law is simply a minor piece of anti-Jewish polemic. If the second, it is a fundamental fact about the role of moral actions in religion.
Deuteronomy 30:19 "This day I call heaven and earth as witnesses against you that I have set before you life and death, blessings and curses. Now choose life, so that you and your children may live" When God calls on us to make existential choices like this, or in the words of Jesus in the sermon on the mount, does this mean we necessarily have the ability to do the things commanded, to make the right choice? Those who say yes, like Erasmus and Rodney Stark and many others, will believe in a Pelagian free will and interpret all the Bible as a law book. Those who say no will have no reason to see what Paul says about the world-wide unshakeable dominion of sin in Romans 3:9-20 and Galatians 3:22 and the bondage of the will in Romans 7 and 9 as hyperbole and "exaggerating to make a point."
If someone could show me how nominalism can change your belief about what Paul was talking about in Romans 7 or the precise referent of "Law" in Galatians, then I would be willing to see it as crucial to the Augsburg Evangelical Reformation. Otherwise, not.
I've provided some follow-up for the points made here in these posts:
Who Is "The Man of Romans 7"?
More on Luther, Augustine, Jerome, and Paul's Epistles
Jerome and Augustine Again
Is Circumcision Sinful in Itself for a Christian?