Background causes:
The Roman Catholic Church's theology was not relating well to the common people. Masses were done in Latin, which common people did not understand. There had also been a loss of confidence in the church through the years of the Black Death in the 14th century when the church preached judgment more than comfort to people during that crisis. And a large number of the church's leaders had become corrupt and worldly, seeking wealth and power and using their spiritual authority to advance themselves at the expense of people.
Trigger event:
Indulgences, which were papers from the pope granting release from penance owed for sins, were being sold to pay off church expenses related to an archbishop buying his position and the pope building a basilica. Martin Luther objected with 95 theses against the sale of indulgences, which were circulated widely and caused a firestorm of reaction from Rome.
Another background cause:
Kings and princes had been establishing their own power, separate from the church. During the Reformation period, princes such as the Electors in Saxony (where Luther lived) were not afraid to stand up for rights over their own subjects who were being threatened by Rome. Or another example, Henry VIII of England was not afraid to take control of the English church into his own hands, away from Rome.
Effects:
A division of the church into many factions occurred -- the Lutheran church in Germany and Scandinavia, Calvinist movements in Switzerland and France, the Church of England as an independent body, plus Anabaptists and other divergent groups.
Political revolutions followed, such as the Peasants War in Germany and the English Civil War in England.
The Roman Catholic Church responded with its own effort to clean up internal abuses and reinforce Catholic teaching -- known as the Counter-Reformation or Catholic Reformation.
