This article from Answers in Genesis gives a informative history of the reformation and its key players. Some might ask why this happened. The short answer is that the Church in that day and age was becoming more and more corrupt. Sadly, in many places around the "civilized" world, the word of God (the Bible) and a proper interpretation was no longer the foundation for the church and its clergy. Thankfully people like Martin Luther felt called by God to reform the church.
Verses for Consideration
Ephesians 2:8-9 ESV
For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast.
Titus 1:9 ESV
He must hold firm to the trustworthy word as taught, so that he may be able to give instruction in sound doctrine and also to rebuke those who contradict it.