The statue at the Martin Luther King Jr memorial by the Tidal Basin in Washington, D.C. in August 2011. (Marvin Joseph)

Fifty years ago, Martin Luther King, Jr. was finishing up the “Letter from Birmingham Jail.” The “letter” was his answer to eight white clergymen, among the most prestigious, racially moderate clergy in Alabama, who had branded King an extremist and condemned the protests roiling that city of fierce racism.

Despite the often calm refutation and allusions to Martin Buber, King’s prison jottings would acquire prophetic intensity. And yet the chiding prophet did not emerge in a flash.

“God is raging in the prophet’s words,” wrote King’s friend, Rabbi Abraham Joshua Heschel. Yet King sometimes seemed more anguished than zealous. In the weeks before he went to jail, he had become weak in spirit, and it was black diffidence, not white hatred, that first discouraged him. The overwhelming majority of black churches, and many ordinary black people too, had turned a deaf ear to him.

It was one thing to sing, “I’m on my way to Canaan land,” another to walk out the church door and risk the wrath of the diabolical Bull Connor. Another verse, “If you don’t go, don’t you hinder me,” hinted at the nagging fear, “what if there were giants in the land of Canaan?”

Back in March of that year, King had told his Ebenezer congregation about a white man who wrote that freedom would not come “until God gets ready. Negroes should stop protesting and start praying!” If blacks believed that, King warned, “we’ll be living in segregation . . . 300 years from now.” The children of Israel, King preached, “felt that if they were to get to the promised land they only get there by God’s power.” But God had told Moses, “Go back and tell them something for me. . . wherefore cryeth they unto me. . . tell them to go forward. . . . Don’t sit around waiting for me to do it all by myself!”

With the protests finally launched in early April, the blacks of Birmingham still did not go forward. As the fear of failure mounted, a testy, at times bitter King can be heard at mass meetings castigating disengaged blacks as “unfit to be free” and “traitors to their race.” This was the anxious mood that engulfed King when he decided to violate the injunction against demonstrations.

Despite the Exodus imagery, the Passion better captured the meaning of King’s decision to go to jail on Good Friday (April 12). He would suffer so that others could be redeemed. How often had he declared, “They can put you in jail and transform you to glory”? This time, as he lay in the darkness of his jail cell, he plunged deep into panic and despair.

But King’s mood would soon take a sudden swerve. Opening a contraband newspaper, he came upon the clergymen’s statement criticizing him and the protests. Suddenly, he was hurtling up out of the valley on a tide of indignation.

The zealous prophet did not show himself immediately in the “Letter.” First came the dance of diplomacy. At the midpoint, however, King declared abruptly, “I must make two honest confessions to you, my Christian and Jewish brothers.” This was a decorous hint of the furious truth-telling ahead. Mainly done explaining himself to white people, King was ready to chastise them.

King launched a glorious array of smack-downs. He berated not the Klan but the ordinary American who was “more devoted to order than justice,” who “paternalistically believes he can set the timetable for another man’s freedom,” who always tells the Negro, King wrote, “to wait for a more convenient season.” Few whites, King judged, had the empathy to grasp the afflictions “of the oppressed race.”

Next the white church came into King’s sights: all those clergy who “sat on the sidelines” and mouthed “pious irrelvances and sanctimonious trivialities.” What really galled King were the ministers who lauded law and order but did not say obey desegregation orders “because the Negro is your brother.” Were they even genuine Christians? On and on, the rebuke flowed down like waters. “Perhaps,” King concluded, “I must turn my faith to the inner spiritual church . . . as the true ecclesia.”

Telling someone they lack the true ecclesia isn’t exactly casting aspersions on their mother. It lacks the bite of King’s first civil rights speech in Montgomery, “‘Be still and know that I’m God, that if you don’t obey me I will break the backbone of your power . . . Standing beside love is always justice.” But the substance was the same: in the main, whites were complicit in racism; their appalling silence was a sin. Here was a refined man’s version of jeremiad, a Christian version of “telling the man.”

Jonathan Rieder, professor of sociology at Barnard College, Columbia University, is the author of the just published “Gospel of Freedom: Martin Luther King, Jr.’s Letter From Birmingham Jail and the Struggle That Changed a Nation” and “The Word of the Lord is Upon Me: The Righteous Performance of Martin Luther King, Jr.”