From his study of the Bible, he was convinced that the church had corrupted its message. For instance, "indulgences" were sold by the church with the promise of salvation through contribution to the building of a cathedral. Luther discovered that the Bible promised salvation through sola fide - faith in Christ alone, not money.
"Unless I am convinced by the testimony of the Scriptures or by clear reason (for I do not trust either in the pope or in councils alone, since it is well known that they have often erred and contradicted themselves), I am bound by the Scriptures I have quoted and my conscience is captive to the Word of God. I cannot and will not recant anything, since it is neither safe nor right to go against conscience. May God help me. Amen."
(from Wikipedia)