What is one of the primary doctrines of the Reformation?

Sam Waldron explains in this video that to identify the primary doctrine of the Reformation, we must begin by looking at the Five Solas. Out of those five, perhaps the cardinal doctrine of the Reformation is “sola fide” or “faith alone” for justification.

Without faith, we cannot please God. Without faith, we cannot be saved. We cannot earn salvation or good standing with God. We are only justified by faith through Jesus Christ. Salvation is a gift - it is something that no one can boast in. 

Hebrews 11:6 (NKJV) – “But without faith it is impossible to please Him, for he who comes to God must believe that He is, and that He is a rewarder of those who diligently seek Him.” 

When I'm asked about the heart of the Protestant Reformation, I think you have to begin by thinking of the five solas, scripture alone, grace alone, Christ alone, faith alone, and the glory of God alone. But if you force me to pick among those five solace, I would say that the cardinal doctrine of the Reformation is sola fidei, or faith alone. The reason is that there were Christians before the Reformation that understood Scripture alone. There were debates about it. There were Christians who understood grace alone.

Augustine is famous for his clear statement of the doctrine of irresistible grace. But if you had to, if I had to answer the question, what's the cardinal doctrine of the Reformation, I would say It's Luther's crystallization and articulation of the doctrine of sola fidei, or faith alone for justification. You