What Led Martin Luther To Break Away From The Catholic Church?

What led to Luther's break with the Catholic church was the practice of selling letters of indulgence. It was the claim of the church that indulgences issued by the pope could shorten the stay of a person, or of a relative, in purgatory. Some even granted forgiveness of sins. Selling such indulgences was one of the avenues used by the church in Luther's day to obtain money.
Since Pope Leo X was hard pressed for funds needed to complete St. Peter's Basilica in Rome, he entered into an agreement with Archbishop Albert of Brandenburg, Germany, that would help with the project. He accepted from Albert the sum of 10,000 ducats in return for the archbishopric of Mainz. That Albert might pay back the sum to the financial house of Fugger, from whom he borrowed it, the pope granted him the privilege of dispensing an indulgence in his territories for a period of eight years. Half of the money received would go to the pope and the other half to the house of Fugger as repayment on the loan.
The Dominican monk Tetzel was entrusted with the sale of these indulgences. He and his fellow vendors made extravagant claims in their sales pitch. Among other things they would say: "As soon as the coin in the coffer rings, the soul from purgatory springs."
Luther was infuriated by such claims. He contended that the granting of indulgences without repentance was contrary to Christian doctrine. He argued: "Indulgences are most pernicious because they introduce complacency and thereby imperil salvation." In one of his sermons in 1516 C.E. he said: "To assert that the pope can deliver souls from purgatory is audacious. If he can do so, then he is cruel not to release them all."