Luther said that the word of God in the Bible and the teaching of the Church were inconsistent.
He preached in German not Latin, and he told people to read the Bible for themselves.
The printing press was new at this time.
In Luther's time the printing press was new and German translations of the Bible were available for the first time.
This eventually caused the reformation in the church, and a reformation in the arts, in science, in commerce, and in politics over the next 300 years.