Sproul's thesis asserts that justification sola fide, or "by faith alone," is the essence or heart of the gospel. He writes, "I am convinced, as were the Reformers, that justification by faith alone is essential to the gospel and that Rome clearly rejects it." Sproul claims that when Rome rejected the Reformers' doctrine of justification sola fide at the Council of Trent (1545-63), the Roman church rejected the gospel itself and officially became an apostate body. he continues, "The flap over ECT is over this very point: the recognition of Rome as a true church despite its view of justification." Sproul seems to be arguing that a church body must subscribe to justification by faith alone as an article of faith, in order to be, in fact, justified by faith alone, since the context of his statements is the salvation status of those who do not believe the doctrine. Sproul claims support for his position from the Reformers…
Sproul supports his thesis from Reformation sources, but his conclusions are not informed by an engagement with patristic and medieval treatments of justification; this is one of the major weaknesses of the book. he does introduce Augustine and Aquinas into the conversation to establish that they believed justification to be exclusively by grace, and he uses their theology to accuse the Council of Trent of semi-Pelagianism.19 Beyond this, Sproul does not substantially treat the views of Augustine or Aquinas on justification. If he had, his thesis would surely have led him, as it did the Reformers, to deal with the question of the Christian status of the pre-Reformation church, since Augustine and the rest of its theologians did not teach that we are justified sola fide in the Reformation sense. In fact, unless Sproul's thesis is qualified, it would lead to the unintended consequence of consigning to perdition the entire Church from the patristic period up to the dawn of the Reformation, something the Reformers did not do. This is because the Reformation understanding of justification sola fide was unheard of in the pre-Reformation church and thus not believed until Luther. Alister McGrath points out that "there are no 'Forerunners of the Reformation doctrines of justification.'"
To put it another way, Luther's doctrine of justification sola fide was not a recovery but an innovation within the Western theological tradition. What is provocative about Sproul's thesis is that the equation of the construct of sola fide with the gospel itself would mean that the Roman Catholic Church not only rejected the gospel at Trent, but the Church never possessed it at all from the post-apostolic period up to the time of Luther. In this unqualified form, Sproul's thesis would also mean that since no one knew the gospel in the pre-Reformation church, no one experienced justification, and thus there was no Church.
Read the whole thing here. Read David Waltz blog posts on this matter here and here. I'm delighted to have come across Mr. Waltz's outstanding blog!