A TESTAMENT OF HOPE. The Essential Writings of Martin Luther King Jr. Edited by James Melvin Washington. Harper & Row. 653 pp. $22.50.

ONE DAY IN June 1968 in the mud and the rain and the confusion at the Washington campsite of the Poor People's Campaign -- a protest of the American poor (Native Americans, browns, blacks and whites) conceived by the Rev. Dr. Martin Luther King Jr., but executed after his death -- a longtime King supporter observed: "This thing lacks vision and spirit and it also lacks an idea. If he'd lived, Martin would have supplied all of those things, but the most important would have been the idea."

That judgment was correct. This collection of King's writings, lovingly edited by James Melvin Washington, a minister and an authority on Afro-American religious history, demonstrates powerfully that in his lifetime and in most memories, King was underrated. That is a strange thing to say about the first American since George Washington whom the nation has honored by designating a special remembrance day. But our memories of King are generally incomplete. We remember him mainly as a brave, eloquent and innovative leader of black people whose human frailties were revealed on J. Edgar Hoover's dirty little tapes.

But beyond all those things, as this book richly demonstrates, King had a piercing intellect which he employed relentlessly in examining, analyzing and criticizing a nation that had wronged him and his people, but which he loved with a Christian passion. It is not surprising that King's intellect is largely unremarked since our pictures of him are mainly filtered through white lenses. Whites have a hard time ascribing intellectual gifts to blacks, who are often described as eloquent, passionate and articulate, but rarely as acute, reflective, analytical or brilliant. King was brilliant.

But what is surprising -- astonishing, even -- is the mass of the written legacy of King's brief public career, which began in December 1955 and ended in April 1968. This book contains speeches -- some clearly off-the-cuff and others crafted with consummate care -- numerous essays, sermons, interviews and selections from the five books he wrote in that period. Considering the demands on his time as the executive of the Southern Christian Leadership Conference, a participant in hard and dangerous civil rights campaigns, the leading spokesman for the black cause, a negotiator and planner, as one among a number of black leaders and as a true international celebrity, the volume and quality of this intellectual work is breathtaking.

Whoever said that all theory is autobiography could have taken as inspiration the life of King's mind. His writings reflect an intellectual struggle and growth that are as fierce and as alive as any chrnicle of his political life could possibly be. Although he was born a middle-class American, the circumstances of King's early life were surely constricting. He was born in the depression and in segregation to a family which had produced preachers in three previous generations. King's father was an overpowering, fundamentalist Baptist minister.

The younger King's earliest writings -- his narrowest -- can be characterized not just as descriptions of the Montgomery bus boycott and elucidations of his nonviolent philosophy, but also as an urgent search for intellectual tools to break out of the fences that surrounded him. He laid hands on the philosophy of nonviolence and the ideas of Tillich, Gandhi and Niebuhr, among others, as crowbars to break out of fundamentalism, segregation and oppression. These early writings have nothing of the power and the sweep of his later vision. Thus, in an article in Christian Century, which appeared in February 1957, King wrote very much as a man of the South.

"It is commonly observed," he wrote, "that the crisis in race relations dominates the arena of American life. This crisis has been precipitated by two factors: the determined resistance of reactionary elements in the South to (the school desegregation decision) and the radical change in the Negro's evaluation of himself."

The "radical change" King perceived was that of the dignity and courage of Montgomery's blacks as they carried out their bus boycott. The power of nonviolence -- the philosophy to which he was to adhere throughout his life -- was deployed in this and other early works as an instrument to crack southern segregation and to move the southern black church out of a posture where God's love merely sustained its people into one in which that love imposed an obligation of service through massive and disciplined crusades for justice. In his attempts to mobilize the enormous potential of the black church, King came closer to attaining a dream he shared with thousands of other black activists and thinkers than any other black leader in history.

As he grew, King began to include the social impotence of the white church in his list of concerns. Then, with his thought under siege from segregationists, other black leaders, the concept of black power, racism in the North and the poison of the Vietnam War, King's vision flowered and grew -- always from the twin seeds of Christianity and nonviolence. His letter from the Birmingham City Jail was not only a challenge to the white church, but has become a classic defense of civil disobedience. His later insights into American racism, contained, for example, in his final presidential address to the Southern Christian Leadership Conference and in his 1967 book, Where Do We Go From Here? are still fresh today -- and, incidentally, contain lessons that could instruct contemporary black conservatives who seem to believe that they have invented the idea of black self-help.

His speech breaking silence on the Vietnam War (April 1967) contains a timeless lesson about the value of dissent in a free society and about America's role in the world.

"Now, it should be incandescently clear," King wrote, "that no one who has any concern for the integrity and life of America today can ignore the present war. If America's soul becomes totally poisoned, part of the autopsy must read Vietnam. It can never be saved so long as it destroys the deepest hopes of men the world over. So it is that those of us who are yet determined that America will be are led down the path of protest and dissent, working for the health of our land."

No opponent of the covert war in Nicaragua or of the proposal to engage in one in Angola could have said it better.

ALL OF THE famous works are here: "The Letter From the Birmingham City Jail," "I Have a Dream," the Nobel Prize acceptance -- but the last SCLC presidential speech shows just how far King's thought had moved since Montgomery, where he had faced only segregation. By that time he had struggled, with little success, with northern racism in Chicago's ghetto and that lesson was clearly much on his mind as he spoke:

"The movement must address itself to the question of restructuring the whole of American society. . . . We must ask the question, 'Why are there forty million poor people in America?' And when you begin to ask that question, you are raising questions about the economic system, about a broader distribution of wealth.

"What I'm saying to you this morning is that communism forgets that life is individual. Capitalism forgets that life is social, and the kingdom of brotherhood is found neither in the thesis of communism nor the antithesis of capitalism but in a high synthesis . . . that combines the truths of both."

Starting from the narrow structures of Baptist fundamentalism and southern segregation, King's mind had battled through wave after wave of orthodoxy and opposition to cut past he two prevailing ideologies of his time to a deeper and richer vision of his beloved America. He had become not just a champion of southern blacks, but of all of America's poor and a force for peace and American decency in the world as well.

The King supporter in the mud of Resurrection City was right. King would have had a better idea for the Poor Peoples' Campaign and indeed a better idea for this era, where the Gramm-Rudman-Hollings budget bill passes for political wisdom. But his ideas would have troubled the nation, for they flowed from a mind that could not abide the status quo, not even its own.