THE AGE OLD TRUTH REALIZED
Upon His death and resurrection, or before it, there was a realzation... Jesus came to free mankind from the Law, not to end it to liberate him from righteousness, but the consequences that came with it.
Peter and the original Apostles in Acts 15:1 & 10, and Paul in Romans 3:19-20 understood that salvation by the law is impossible. But why the law in the first place? It is to close every mouth and make all guilty of it. Therefore, Paul said. "... by the deeds of the law no flesh shall be justified for through the law comes the knowledge of sin".
And so what is the solution to the fallen man? By mere faith (Romans 3:23-26).
In Augustine's time the church was in agreement with him against Pelagius who forwarded that salvation is attainable because man is unincumbered by sin and so has the power to save himself, In Luther's time however, Rome has imposed more than the law as the means to salvation but by the sacraments and sales of indulgence as well.
He was deeply troubled by the abuse of the latter but still followed the commandments fearfully until he had read Romans 1:16 and realized that salvation is given and powered by God to save the person, and not the person himself.
WIth that rediscovery, and the nailing of the 95 Theses on the Wittenberg door, the Reformation Movement began.
THE IMPORTANCE...
... of this doctrine cannot be stressed enough. Our own works cannot save us because to go to heaven, one must be perfect as God is. We are already condemned because of our sins but to live the law as a means of salvation only ensures it as stated in Romans 4:4.
And if there is only one way to heaven, that means there is only one Gospel (Galatians 1:6-9), and the Catholic church teaches a false gospel (Galatians 5:4).
The Reformation movement did not merely to reform the Catholic church for it's unbiblical traditions but to end the false gospel that can only condemn.
There are other churches that considered themselves as Protestant because they too protest the unblblical practices of the Roman Catholic church, but the same also teaches a false gospel. They are not Protestants.
Our mission as Protestants is to not only expose the errors of the Catholic church lovingly, but to share the true Gospel to them (as also those against Sola Fide). In fact, of all errors to be illuminated, the Gospel is the most important. They're not saved by casting out statues and Rosery beads, but by accepting Christ alone by faith alone.