What was one of the most impactful reforms of the Reformation?

Sam Waldron explains in this video that one of the most impactful reforms was Martin Luther’s clear articulation of the doctrine of justification by faith alone. We are justified by the righteousness of Christ alone when we rest on and receive Christ by faith alone.

We must distinguish between that initial justifying faith and the obedience that flows from it. Our obedience is the fruit of justification but not part of it or the basis of it. There is nothing that we can do that can merit salvation.  

Ephesians 2:8 (NKJV) – “For by grace you have been saved through faith, and that not of yourselves; it is the gift of God.”

I think one of the most effective and important reforms was Martin Luther's clear articulation, along with the other early Reformers, Kelvin and Zwingli, was Martin Luther's clear articulation of the doctrine of justification by faith alone. That we are justified by the righteousness of Christ, an alien righteousness, an external righteousness, and we are justified by that righteousness when we rest on or receive Christ, that is to say, by faith alone. And that we must distinguish between that initial justifying faith and the obedience that flows from it. We are justified by that initial justifying faith. The obedience is the fruit of justification and not a part of it or the basis of it.