Two most important things that

Luther had a problem with

The fact that their modern-day catholics said that sins were needed to be forgiven through a priest or some form of higharchy in relation to the rest of the people, so basically that Catholics at that time believed that there was not a direct relation from God to mankind. And two, that in order for you sins to be forgiven, you had to pay these more royal people. Both of these went completely against what is in the bible.

I hope you are not talking about Martin Luther King Jr., because I am talking about the Christian Reformation!

One was social rights and discrimination