Martin Luther disagreed with many things Catholic, but the main things were the purchasing of salvation or earning salvation by performing good works. He taught (which I believe as well) that salvation can not be earned at all, but only by the grace of Jesus Christ. "For by grace you have been saved, and that not of [good works], lest any man should boast". The other thing he disagreed with was that the popes did not have "divine right". I hope this helps! :)