“The most fatal blow to progress is slavery of the intellect. The most sacred right of humanity is the right to think, and next to the right to think is the right to express that thought without fear.”

"Suffrage" the right to vote, was a monumental task for women to gain. Seeing the fear in men's eyes as the streets burned in England. The jig was up, and the power of suppression through religious dogma was revealed. Yet the power to control, influence base desires of men was unleashed in the halls of government. The most manipulative force on planet earth, the ability to arouse base desires of the carnal mind and body. 

I'm not sure I would say that: "The most fatal blow to progress is slavery of the intellect" but, I would certainly say it is at the top 2 or 3 of any list that describes progress inhibiting (spiritual, mental, moral, physical, economically, etc., etc., etc.).