
He objected to indulgences as buying one's way into Heaven, and he objected to where the money was going: out of Germany, for rebuilding St. Peter's Basilica in Rome.
His theses circulated widely, but the Catholic Church decided that they were heretical, and a few years later, the Church put him on trial at an assembly in a town called Worms -- the Diet of Worms. He refused to back down, and the Church excommunicated him. But some leaders befriended him and supported him, as he established a new church, translated the Bible, composed hymns, wrote about various theological issues, and got married to an ex-nun.
He hoped that his new improved version of Christianity would make Jews want to convert, but when they failed to do so, he turned against them. He wrote "On the Jews and Their Lies" (1543), advocating that Jews be either expelled or turned into slave laborers. Some 400 years later, many Nazis loved what he said about Jews and what to do about them.
He was followed by several other reformers, like John Calvin, and some of them also established their own churches. The Catholic Church did not like these new churches, and the result was some 150 years of bloody Wars of Religion. These wars ended in a draw, with much of northern Europe Protestant and the rest Catholic.
Martin Luther succeeded where most of his predecessors had failed. A century earlier, reformer Jan Hus (John Huss) had some success in Czechia, but the Church caught up with him and burned him at the stake. Even earlier, the Church successfully suppressed the Albigensians of southern France, and before that, the Bogomils of Bulgaria. The Eastern Orthodox Church, however, was too big and distant for the Catholic Church to suppress, however.
Although his immediate legacy was those Wars of Religion, his further legacy was the weakening of the Catholic Church and the growth of religious tolerance and freedom of thought more generally. Catholics and Protestants discovered that they could coexist, despite considering each other idolators and heretics.