What German monk believed that Christians had to live by faith alone and depend on the mercy of God?
B. Martin Luther.
Martin Luther was a German reformer who expressed his opinions regarding God, faith, salvation, the Indulgences' sale, and the Church in his 95 theses known as "Disputation on the Power of Indulgences."
He believed that salvation was not decided by the person, but by the grace of God. Faith alone dictates and justifies a person's actions, and thus, grants their salvation.