It is important to remember that Martin Luther did not originally want to leave the Catholic Church, he wanted to reform it. As a result, he was forced out of the Church. Luther, a monk and former law student, was appalled at the corruption he saw in the Church. He saw Church leaders living immoral lives and using the religion to increase their wealth. What particularly pushed Luther to speak out was the selling of indulgences. These were payments that Catholics could make to the Church to absolve sin and limit the time they or someone else spent in purgatory. Luther saw this as a blasphemous moneymaking scheme that took advantage of faithful but naive Catholics.
In 1517, Luther publicly stated his complaints against the Church when he posted his "Ninety-Five Theses." In this, he denounced the sale of indulgences and explained the biblical source of salvation. He publicly urged Church leaders to abandon their corrupt practices. This was a dangerous move on Luther's part. Previous reformers had been declared heretics and executed.
The Church ordered Luther to recant, but he refused. At the Diet of Worms in 1521, Luther was excommunicated from the Catholic Church and declared a heretic. Forced out of the Catholic Church, Luther went on to start his own separate denomination known as the Lutheran Church. This new denomination sought to abandon the worldly trappings and the non-biblical tenets of the Catholic Church.