The invention of Imputed Righteousness
McGrath explains that the criteria used throughout the Reformation period to distinguish Protestant from Catholic was the question of whether justification was forensic (ie used a legal, court-based, paradigm). (pg 215) McGrath argues that the concept was fundamentally new within the Christian theological tradition, as was the Protestant separation of sanctification and justification.
"Luther... introduced a decisive break with the western theological tradition as a whole by insisting that, through their justification, humans are intrinsically sinful yet extrinsically righteous." (pg 213, cf 217) "The significance of the Protestant distinction between iustifcatio and regeneratio is that a fundamental intellectual discontinuity has been introduced into the western theological tradition through the recognition of a difference, where none had previously been acknowledged to exist." ... "The Protestant understanding of the nature of justification thus represents a theological novum".
Interestingly, McGrath explains that the Protestants at the time vehemently denied that their doctrine was new and unprecedented. Melanchthon claimed he was returning to Augustine's teachings on justification. McGrath explains that in reality these Protestant claims were entirely without basis and that the Catholics were Augustinian and Melanchthon was departing from Augustine. (pg 216)