What was a driving force behind the Reformation?
Joel Beeke explains in this video that one major driving force behind the Reformation was the doctrine of justification by faith alone. Once Luther, Calvin, and the other Reformers grasped hold of the doctrine of justification by faith alone, the Roman Catholic Church’s false teachings were exposed. A person's supposed "good works" could never save a sinner.
The Reformation soon became known as a reformation of grace alone - that grace alone saves us. While we are sanctified and cooperate with the Spirit in our sanctification, there is never any credit due to us or anything we that must do to "earn" our salvation. We are saved by God’s grace alone.
Ephesians 2:8-9 (NKJV) - "For by grace you have been saved through faith, and that not of yourselves; it is the gift of God, not of works, lest anyone should boast."