The Social Gospel movement, associated above all with Walter Rauschenbusch's writing in the early twentieth century, argued that Christian ethics obliged the church toward structural reform of unjust economic and social conditions, not only individual charity and personal salvation. It supplied the theological premise later movements needed, that sin is social as well as personal and that faith owes the world change, without settling what method that change should take.
Mohandas Gandhi supplied the method. His practice of satyagraha, developed first in South Africa and then in the Indian independence movement, combined public noncooperation, willing acceptance of suffering, and strict nonviolence into an organized tactic rather than a personal virtue, and Gandhi read it partly through the Sermon on the Mount even as he built it from Hindu and Jain sources of his own. His own account of the method's development remains the primary record of how deliberately it was engineered as strategy rather than merely temperament.
Reinhold Niebuhr complicated the inheritance rather than simply extending it. In Moral Man and Immoral Society, 1932, he argued that individual moral suasion, the Social Gospel's usual tool, cannot on its own dislodge entrenched group power, and that nonviolent coercion, forcing an opponent's hand through organized pressure rather than persuading their conscience, was the more honest description of what effective nonviolent campaigns actually do. That argument reached the American civil rights movement directly. Organizers who studied both Gandhi's tactics and Niebuhr's realism built a Christian nonviolent witness meant to compel change, not merely to model virtue and wait.