Martin Luther believed that faith was the key to salvation and being allowed into Heaven. He believed after studying the Old Testament and after what he claimed was a visit from the Holy Ghost that sincere faith, which was a gift from God, was the true path to being saved. The idea that people could only obtain salvation through good works was not something he agreed with because there was no way for a person to know if they had been given grace or salvation through their works.
It was this basis that lead to Luther breaking with Rome, but it was just the start of his ideals. He spoke out against the selling of indulgences and against the fact that the Pope or any other church official had actual power over what happened to a person's soul. They were just simple men, and he believed that anyone could read the Bible and gain God's message. In his opinion, it did not take any special gift or ability to communicate with God, just the simple desire to do so. Luther also believed that any ritual, belief or practice not found in the Bible should not be allowed. The latter of these led him to reject the veneration of Saints, fasting and the entirety of the Papacy.