Why was Martin Luther such a threat to the RCC?

Martin Luther and his ideas spread quickly, due to the invention of the printing press. He did not condone indulgence selling as a method of getting out of purgatory and therefore the RCC were set to lose a substantial amount of money if they were forced to stop selling them. Later Martin Luther became a threat at Lutheranism developed. This created off shoots of the RCC and divided them, making them create the Council of Trent and begin the counter reformation