The clash between German cleric Martin Luther and Pope Leo X is the central drama in the 16th Century religious revolution known as the Reformation.
The clash between German cleric Martin Luther and Pope Leo X is the central drama in the 16th Century religious revolution known as the Reformation.
Luther's stand against what he saw as Catholic Church rituals and practices that were not supported by the Bible led to a religious and political crisis that split Christian Europe in two.
The division triggered a series of religious wars across Europe and indirectly led to America's earliest colonies being dominated by people seeking freedom to practise their beliefs free from persecution.
By the beginning of the 17th century, England, Scotland, the states of northern Germany, Denmark, Norway, Sweden and large parts of Switzerland were predominately Protestant. France and southern Europe remained Catholic.
Luther, born in 1483, was not the first to call for wide-reaching reform of the Catholic church. For centuries would-be reformers had railed against what they considered to be church corruption.
Many had been persecuted and excommunicated by the Pope - a move that effectively denied them access to the church and placed them outside the law. Some were burned at the stake for heresy.
The reformers were particularly enraged by two practices they believed were corrupt.
The first was simony - the selling of high-ranking positions within the church to the highest bidder. This regularly led to the teenage sons of powerful aristocratic families being appointed as bishops and archbishops.
The second was the selling of indulgences. Officially an indulgence was an acknowledgement by the church that a worshipper had done penance for a sin and been granted absolution. But unscrupulous clerics were known to sell indulgences that promised the forgiveness of sins and "smoothed the path" for entrance to the kingdom of Heaven.
Many reformers also questioned the Papacy's claim that the Pope ruled by "divine right" and that his every utterance was inspired by God.
By Luther's time there was also a fierce debate over many aspects of church ritual that had a central role in Catholicism. These included the importance of Mary and the saints, the refusal to allow priests to marry or have children and the use of wine and bread as symbols of Christ's blood and body during Mass.
Many reformers saw these as little more than superstitions imposed by the Catholic Church over the centuries.
They called for a "cleansing" of the church - and the introduction of a new system based strictly on the Bible.
On October 31, 1517, Luther made a very public stand. He nailed a document known as the Ninety Five Theses to the door of a church in Wittenberg in the German state of Saxony.
His writings roundly criticised the Church and the Pope and set out the arguments against what he saw as the worst abuses. It was nothing short of a declaration of war.
Luther's writings rapidly circulated through northern Europe, the heartland of the fledgling reform movement.
Pope Leo X fought back. His theological experts studied Luther's writings and charged him with heresy. A year after he nailed his theses to the door Luther was summoned to the city of Augsburg to answer for his alleged crimes.
During a bad-tempered debate with Leo's envoy Cardinal Cajetan, Luther first refused to disown his writings - and then refused to recognise the papal authority. He fled the city a few hours later.
In January 1521, after Luther again refused to back down, he was excommunicated. Two months later he was summoned to appear before a council — or Diet as it was known in German — of the rulers of various states that made up the vast Holy Roman Empire. The council met in the city of Worms on the River Rhine in Germany.
Luther was asked if he stood by his writings. He is said to have replied: "Here I stand. I can do no other."
On May 25, 1521, the Diet of Worms declared Luther an outlaw. This meant it was illegal to give him food or shelter. Anyone who killed him would not be punished.
But Luther's revolution was unstoppable.
In northern Europe support for the reformers - who became known as Protestants because of the protest against Rome's rule - continued to grow.
Luther himself returned to Wittenberg. From the mid-1520s he devoted himself to organising a new church doctrine based on Bible teaching, which became known as Lutheranism.
Central to it was the idea that Christians do not achieve salvation by doing good in their lifetimes - but through "the grace of God."
He also translated the Bible from Latin into German so it could be read by ordinary people.
In 1525 he married Katharina von Bora, setting an example for other reforming clerics to abandon the Catholic church's vow of chastity.
Luther's later years were dogged by ill health, particularly heart problems. He died in February 1546.
Reformers elsewhere in Europe were inspired by Luther but set up their own Protestant churches after differences of opinion on matters of theology.
This led to the growth of other movements such as the more radical Calvinists, Anabaptists, Puritans and Presbyterians.
The Puritans were later among those who founded colonies in the New World so they could be free to practise their religion.
Ironically, Luther's revolution did lead to change within the Catholic church. Between 1545 and 1563 the Vatican held The Council of Trent, which addressed many of the issues raised by the reformers - while still declaring them heretics.
But the upheaval sparked when Luther nailed his writings to the door of the Wittenberg church also led to a series of religious wars across Europe.
The most destructive was the Thirty Years' War, which began in 1618. Fought by rival Protestant and Catholic armies, it devastated Germany. It has been estimated that eight million people died - about a third of the population of the German states.
