This biographical sketch is part of the article:
Why the Reformation? (T&T 2001) by A E Bouter
After considerable formal schooling and a frightening experience in a thunderstorm, Luther entered an Augustinian monastery in 1505. Staupitz, the vicar-general of the Augustinian Order, encouraged Luther to study the Bible, which he did with zeal.
In 1510-11 he was sent to Rome by his monastic order, and there he saw for himself the corruption and luxury of the Roman Church, things he would never have seen in his native Germany (Saxony).
After this, he was appointed as a professor of theology at the University of Wittenberg, and there he began to lecture his students using the Bible as his text. It was there, around 1513-17, that he understood and accepted that justification of a sinner by God comes through faith, and by implication not from the clergy through the sacraments or by one's own efforts.
The Bible became the only authority he trusted, after a study of the original languages.
When Tetzel, the indulgence seller came into the area near Wittenberg, Luther boldly opposed him (with the support of the secular ruler, the Elector of Saxony, but in defiance of the Pope).
Originally Luther only called for a reform of the indulgence system. But by 1521, both he and Pope Leo X realized a complete break between Luther with his ideas and the Roman Church would take place. Luther was given opportunity to recant, but he refused and was excommunicated. Luther declared he would recant if the Bible could prove him wrong. This did not happen.
In the meantime, Luther's ideas were being spread and receiving wide acceptance in Germany. In a series of writings, Luther attacked the papal hierarchy, the sacramental system (rejecting that salvation must come through the sacraments), and the theology of Romanism (which had replaced the priesthood of all believers). Luther soon translated the Bible into German, and it was widely read by the German people. In 1529, following the second Diet (council) at Speier, that a group of Lutheran nobles wrote a 'protestation' to the decisions of the Diet (which had been led by Roman Catholics) and the word 'Protestant' came into use.
Luther, with the help of Philip Melanchthon and others, gradually developed what is known historically as Lutheranism (though many Lutherans today appear to have no firsthand knowledge of Luther or his theology).
 Not with the full biblical light as we know today, but still it was a tremendous step with God.