What is the Reformation, and what did Martin Luther and John Calvin do in the Reformation?

The Protestant Reformation lasted from 1517 to 1648.
During the Middle Ages, the Roman Catholic Church was an extremely powerful, unifying
force of the people. As a result, the pope acted as the intermediary between men and
God. As the Church gained more and more power, it began to use corrupt methods to earn
money and control the mass. When the Church need to finance its large building projects
for Renaissance artists, it sold indulgences. Indulgences were “tickets” that saved
humans from going to the purgatory. Land owning nobles’ wealth and power were being
exploited as the Church grew. This set the stage confrontation of those called
Protestants to the corrupt nature of the Catholic
Church.

Martin Luther, a German monk, led this movement
with a group of followers calling them Lutherans. Luther nailed a list of 95 Thesis on
the church door, outlining his frustration with the current Church corruption. He
declared that salvation was not granted by indulgences, but through grace.This
revolutionary concept reduced the role of the Church as an exclusive intermediary.
Luther’s ideas spread widely, with the help of the printing
press.

Lutherans began to separate themselves from the
Catholic Church and developed a new theology asserting their own biblical
interpretations. John Calvin led a strong Protestant group by preaching “predestination”
stating that God had predetermined an ultimate destiny for all people, and only those
who preached would be saved.