95 Theses, this represents Luther's attempt to sway the church out of misuse.
Luther was the first person to stand up against the church and their corrupt indulgences. Luther and many others believed absolution was earned through faith and positive deeds done during one's lifetime. The church was simply selling indulgences with absurd marketing terms such they will grant dead ancestors absolution. Luther's 95 theses directly discuss the corrupt and morally wrong ways the church used indulgences. He is a saint in this aspect as he is preserving christian ideals and swaying it back out of misuse.
Pope Benedict XVI, wearing hundreds of thousands worth of clothing and gold.
One major idea that makes Luther a saint is his understanding of how authority in the church SHOULD work. Luther witnessed priests, cardinals and the Pope granting forgiveness and damning people. In true Christianity, God is the only to give forgiveness, God and Jesus Christ have ultimate authority, not the Pope or anyone within the church. This picture depicts a pope in what Luther would probably consider excessive, and the pope should not be raised this high wearing clothing worth enough to feed an entire city.
Gutenberg's printing press
With the introduction of the printing press, it was the perfect opportunity for Luther to translate the Bible into his native language. He is a saint for this translation as many that shared his language could not speak Latin, the only allowed language for the bible to be printed in. Anyone who could not read Latin had to go to priests for translation of the Bible, this caused problems as these priests also gave their own interpretations of the Bible. Translating the Bible meant individuals could interpret the Bible for themselves and dedicate themselves more to a faith they actually understood.
Luther posting his 95 Theses on the door of a local monastery.
Luther's 95 These were seen by many as a direct attack on the church and the Papal. Luther was opposing traditions that had been used for decades by the church and his Theses would ultimately cause the church to lose power if implemented. The Pope is greater than normal human, Luther was opposing the Pope therefore making him a sinner.
In the catholic church, the bible and church service was restricted to Latin. Luther directly disobeyed this restriction by translating the bible into German. He is seen as a sinner for this not only because he disobeyed the Church, but he disrespected the religion as a whole by translating the bible into this impure and inferior language. This was seen extremely sinful by many as he was trying to change a tradition that had been followed for centuries before.
Among many of Luther's beliefs, he accepted and changed many beliefs that had been followed for centuries and directly disobeyed the Papal. One of the most controversial is adult baptism. Luther believed in the practice of adult baptism even though it had been opposed by the catholic church for centuries. Luther also believed one cannot lose salvation after receiving it and Luther generally believed the Catholic church has strayed far from traditional Christian beliefs. These were all also considered sins and heretical.