What were Martin Luther’s beliefs that conflicted with the Catholic Church?

Martin Luther began his religious career as a pious Augustinian monk and professor of theology dealing with overwhelming feelings of sinfulness and finding consolation by reading the apostle Paul’s “Epistle to the Romans,” in which, according to his interpretation, he discovered the principle of justification by faith alone (sola fide). This principle emphasized an intense personal relationship between the individual believer’s conscience and God, and it focused on God’s grace, freely given (sola gratia). Luther’s stress on the importance of individual conscience and the direct relationship between God and individual believers made it impossible for him to accept the selling of indulgences, which supposedly absolved the sins of the purchaser in exchange for a sum of money (provided that the purchaser also repented sincerely). The pope used indulgences to finance the building of the new St. Peter’s Cathedral in Rome. In 1517, Luther published his objections to indulgences in the form of 95 theses, and Catholic theologians immediately started to criticize his position.
In response, Luther, who received support from a significant portion of the German population, including many peasants, urban artisans and merchants, university students, some clergymen, and a large percent of the nobility, launched an all-out attack on the idea of the Church as an institution and the clergy as a separate estate mediating between God and secular society. Luther demanded the confiscation of Church estates and asserted the principles of freedom of conscience and the equality of all Christian believers before God. While criticizing many Catholic traditions (including monasticism) as lacking Scriptural support, he made the Bible the main reference point for Protestant theology (sola Scriptura). Luther translated the Bible from Hebrew and Greek into German; this translation came to play a major role in the development of German as a literary language and served as a model for other translations of the Bible into vernacular European languages. Many Catholic Church leaders during this period were very critical of such translations, which they saw as dangerous because of the possibility that readers untrained in Catholic theology might develop and popularize heretical interpretations of the text. Protestants, on the other hand, actively began using the printing press and promoting education and literacy in order to make it possible for more people to access Scripture and Luther’s ideas.
Luther emphasized the role of secular princely authorities as protectors of his new Protestant religion. He demanded unconditional obedience to government and feudal authorities. He stressed the importance of the patriarchal family as the backbone of society. He himself married a former Catholic nun, Katharina von Bora, and had six children from this marriage.