According to Martin Luther, in his preface to the 95 Theses:
Out of love for the truth and the desire to bring it to light, the following propositions will be discussed at Wittenberg, under the presidency of the Reverend Father Martin Luther.....
Contrary to what many think, it was Luther's intention to reform the church, not break away from it entirely. He pointed out 95 things that, after much prayer and study of Scripture (notably the book of Romans), he felt should be pointed out, revised and/or changed. Some of the main things that he criticized was the sale of indulgences, the misleading of the people by self-serving priests and that salvation could be obtained through works. After reading the book of Romans, Luther, inspired by God, came to realize that salvation is by faith through the grace of God, not through any work of the church or the Pope.
If I were to choose 3 of the theses and put them in my own words, I would choose number 32:
32. People who believe that indulgences will let them live in salvation will always be damned - along with those who teach it.
People believed that they could pay money to get rid of their sins, and the priests were encouraging this to get money for the church.
36. A man can be free of sin if he sincerely repents - an indulgence is not needed.
It says in the book of Ephesians that "By grace you are saved, through faith, and that not of yourselves. It is the gift of God lest any man should boast." All that is needed, therefore, for salvation is faith and repentence, not confessing your sins to any man.
42. Christians should be taught that the buying of indulgences does not compare with being forgiven by Christ.
Again, the same idea - that indulgences cannot be bought. Only God can forgive sins.
If you check the 2 links below, you will find the original 95 Theses as well as a modern translation - there are lots to choose from, but I chose 3 that were related in theme.