This Good News tells us how God makes us right in his sight. This is accomplished from start to finish by faith. As the Scriptures say, “It is through faith that a righteous person has life.” Romans 1:17
The church of Luther’s day taught “justification” (being right with God) was a progressive work starting with baptism followed by good works and penance. (Perhaps that’s oversimplifying, but that’s the basic idea.)
Years ago, when my sons were younger, we had a basketball hoop (not adjustable in those days), and one of my sons desperately wanted to ‘dunk the basket’ which at his age and height was totally unrealistic. So, I put him on my shoulders, and he ‘dunked’ the ball. He was thrilled!
Luther was like my son, desperately trying to ‘dunk’ the ball by being ‘good enough’ for justification. He did all the things that the church asked and taught but always fell short.
It was when he read Paul’s words to the Romans (quoting Habakkuk 2:4) that the Holy Spirit broke through to him with the revelation that only by faith could he please God.
Many scholars refer to this revelation as “the material cause” of the Reformation.