I’ve found no verifiable evidence that Martin Luther King, Jr. ever said or wrote that quotation. There wasn’t a verifiable “Steeler lecture,” and there isn’t any instance of that quotation any of his published writings or speeches, including The Trumpet of Conscience or the 1967 “Massey lectures” broadcast and published by the CBC (Canadian Broadcasting Company).

The closest actual quotation to that I could find was by Elie Wiesel, in his forward to “The Courage to Care: Rescuers of Jews During the Holocaust” (1986): “Let us remember: What hurts the victim most is not the cruelty of the oppressor but the silence of the bystander.”