What year did Martin Luther start the Protestant Reformation?

The beginning of the "Protestant movement" is more commonly known as The Reformation, or The Protestant Reformation.

It's not easy to say exactly when Protestantism began (or when the Reformation began), because several things contributed to it, and they didn't all happen at the same time. The most common dating begins in 1517 when an Augustinian monk called Martin Luther brought things to a head by publishing his Ninety-Five Theses (Wikipedia link).

He was protesting against (among other things) the Roman Catholic Church's selling of indulgences. Indulgences were a way of paying money to have sins forgiven, and in 1517 the then pope, Leo X, started selling indulgences to raise money to rebuild St Peter's Basilica (cathedral) in Rome.

Luther reacted against this blatant abuse with his 95 Theses, which he nailed to the church door, as an invitation to start a debate. The church had a big problem with this, because the as indulgences were authorised by the Pope, Luther was effectively challenging the Pope's authority, and saying that the Pope could not forgive sins, only God could. This did not go down well! When Luther repeatedly stuck to his position the church excommunicated him, and he was outlawed by the emperor. He would have been killed, but was abducted and protected by the Frederick 3rd, Elector of Saxony.

The Reformation took some years to get going, and happened at slightly different times in different countries and cities. For example, the English reformation took place between 1529 and 1536.

Protestantism's two most important doctrines or beliefs were (and still are):

1. "sola scriptura" - the Bible, and only the bible, is Christians' final authority in all things. (The Catholic Church believed in the ultimate authority of the Bible AND of the Church. That's why the protestants used the term "sola scriptura" - ONLY scripture, not the church's authority too).

2. "sola fide" - justification by faith alone - which means that we can be forgiven by God and considered righteous by Him not because we deserve it (we don't), but ONLY by faith in Jesus Christ's death on the cross, where he took the punishment we deserved. This belief contrasted with the Catholic belief that our good deeds are also necessary for us to be 'justified' (considered righteous) before God, implying that Jesus Christ's death for us on the cross was not enough.

For further reading, there are several articles on Wikipedia, e.g. see link below:

There are also more easily readable accounts on the internet and in print, e.g. an encyclopeadia should have one.