Martin Luther essentially believed that the only path to Salvation was a person's personal faith in Christ and not any actions that can be approved by the Church.
Explanation:
In his time, the Church would sell 'Indulgences', pieces of paper given in return for charitable efforts, prayers etc
People would collect these 'indulgences' as a way to collect enough in order to guarantee a path to heaven or get some for loved ones.
It is true, that a lot of this money was used to build grand cathedrals there even developed a black-market for the buying and selling of these.
Martin Luther was completely correct in his stance against this practice.
