Martin's parents wanted him to become a lawyer - after all, that would bring his family more prestige, and that meant quite a lot in 16th century Germany. But young Martin had a religious experience - he was on horseback during a thunderstorm and a lightning bolt struck near him. In terror, he cried out "Help! Saint Anna, I will become a monk!"
And he did.
But Luther was not content as a monk - he dedicated himself to monastic life, but was eventually ordered by his superior to teach theology at the University of Wittenburg. It was during this time that the Catholic Church began selling indulgences again - a practice that Luther abhorred, because it implied that you could "buy" your way into heaven by good works, or purchasing indulgences from the church. In 1517, Luther wrote to his bishop, Albert of Mainz, protesting the sale of indulgences. This was his famous '95 theses.' Legend has it that he nailed the 95 theses to the door of the church, but there is little to support this claim.
In 1518, friends of Luther translated the 95 Theses from Latin into German, printed and disseminated them. Within two weeks, copies had spread throughout Germany; within two months throughout Europe.
Luther, in the beginning, did not intend to attack the Church. Like many of his time, he was unhappy with the actions of the Church, and wanted answers. He got his answer: in 1521, he was excommunicated for his writings.
Luther claimed that a personal relationship with God was possible. This was revolutionary - for over a thousand years, the pious had to speak to God through a religious intermediary - a priest. The Church ruled spiritual life - it held the people at it's mercy, because without it's services, one could not enter heaven. The church performed baptisms, performed mass, offered communion, heard confessions, performed marriage rites, and death rites.
In Luther's new churches, the congregation sang the hymns. Sermons were read in German - NOT Latin. Pastors could marry. Luther himself even translated the bible into German so that laypeople didn't need to learn Latin to read God's word.
To put it all into perspective: Martin Luther reluctantly antagonized the largest, most powerful institution of his day - and WON.