Why Compromise on Justification by Faith Marks the Collapse of Protestantism

Why did Martin Luther say that justification by faith is the head and cornerstone of Christianity and if this point falls, all falls? When reformers went to negotiate re-unification with Roman Catholic Church leaders, why did Luther say, “Nothing of this article can be yielded or surrendered”? Why is the doctrine of justification the dividing point between Roman Catholicism and Protestantism?

What has made it possible for Protestant churches to reunite with Rome now? How were the councils of Trent and Vatican II different? What was the same about them?

These questions and others are answered in this study on the doctrine of justification by faith, the cornerstone of Protestant Christianity.