What was reformed (or recovered) during the Protestant Reformation? In the final analysis, it was the gospel of God’s grace.
If one wants to know what the Protestant Reformation was all about without reading huge volumes of historical literature, it is perhaps most clarifying to look at the theological results. One should specifically note the rediscovery of five critical biblical doctrines that had been obscured from public view by the medieval version of what we now know as the Roman Catholic Church. And just so you know, Rome still either openly opposes or seriously distorts these doctrines. Using the Latin names given to each, they are:
Sola Scriptura (Scripture alone):
The Reformers were united in their belief that the Bible alone teaches all that is necessary for salvation and Christian living (cf. 2 Peter 1:1-4). They held the Word of God to be the only standard by which men’s consciences may be bound. Rome, on the other hand, then and now, denies sola Scriptura by elevating Papal decrees and church tradition to what they say are equal (but are in reality greater) positions of authority than that of the Bible. Where the meaning of the Bible differs from the opinion of the Pope or official doctrine (as is very often the case) the Word of God plays a mute second fiddle.
Sola Gratia (by Grace alone):
The reformers understood that salvation is not a cooperative event carried out by God and man working in partnership. In salvation, sinners are rescued from God’s wrath by His grace alone (cf. Titus 3:3-7). God’s grace is His spontaneous and unmerited favor, granted to the spiritually dead and helpless sinner through the regenerating work of the Holy Spirit. God mercifully releases those whom He is saving from their own willful bondage to sin and thus enables them to repent and believe (cf. John 3:3; 6:44; Rom. 8:6-8; 9:16). Interestingly, this point of doctrine is disputed today, not only by Rome, but also by many evangelicals.