<human>:  When did the Protestant Reformation begin?
<bot>: The Protestant Reformation began with the work of Martin Luther in 1517. He was a German monk who became convinced that the Catholic Church had become corrupt. He started a movement to reform the church. He was particularly upset with the sale of indulgences, which allowed people to buy their way out of punishment for sins. He started a movement that would lead to the creation of many Protestant denominations.