Last May the leaders of the Mormon Church startled the world with a biting denunciation of the "terrifying arms race" in nuclear weapons that, they said, were "potentially capable of destroying much of civilization."

What made their statement so surprising was that it marked the first time in recent history that the 151-year-old church, which has made patriotism almost an article of faith, has seriously challenged a major government policy.

Behind the surprising about-face of the Mormon leadership on the nuclear issue was a series of quiet but intense conversations with a number of theologians from main-line churches. The leading figure among the persuaders was John Coleman Bennett, a mild-mannered, 79-year-old churchman who has spent more than a half-century defining, on a global scale, the implications of the Christian mandate to love one's neighbor.

Bennett's involvement is but the latest in the distinguished clergyman's long career of preaching the social gospel. For Bennett, it is unthinkable for the Christian church not to be involved in secular affairs.

"Things happen in the world which force the church out of its ruts and enable it to have a fresh view of its agenda in the light of its own message and mission," he says. Bennett sees the church called to combat not so much personal sins such as sexual transgressions but rather governmental policies that, he believes, deny justice to the poor and threaten the world with nuclear annihilation.

History will surely rank Bennett as one of the giants of Christian thought in his generation. Not strictly speaking a theologian, Bennett's genius has been to apply the theological insights of men like Reinhold Niebuhr, with whom he was intimately associated for much of his active career, to major issues of the day.

Bennett retired 11 years ago from the presidency of Union Theological Seminary in New York, then the leading Protestant seminary in the country, and moved to California. But he still teaches courses in universities and seminaries up and down the California coast. Until two years ago, he commuted once a week from his home in a church retirement community in Claremont to Berkeley to teach at Pacific School of Religion.

In addition, he continues to write and to criss-cross the country on speaking engagements, such as the one that brought him to Washington last week as part of Wesley Theological Seminary's centennial observance.

For more than a half-century, Bennett the teacher has influenced generations of seminarians, and through them the church. He also has practiced what he preaches and his influence has been felt in a variety of issues:

Probably more than any single individual, Bennett helped to mobilize the churches against the Vietnam war, at first by the clarity and persuasiveness of his argument; later by example as both he and his wife, Anne, took to the streets with their students from Union and were arrested in several antiwar demonstrations.

In 1960, as John F. Kennedy's run for the presidency began warming up, Bennett took on much of the American Protestant community, including such popular figures as Bishop James Pike and Norman Vincent Peale, and insisted that the presidency of this nation must not be barred to any man because of his religion.

Six years later he spearheaded a group of 85 religious and scientific leaders who petitioned Pope Paul VI to take an affirmative stand on birth control and the need to limit population.

When the 1970 student riots at Columbia University spread across New York's upper Broadway to Union Seminary, Bennett responded so amiably to student demands that leaders of the would-be rebellion complained that there was nothing left for them to fight against: "It's like trying to fight a bowl of Jell-O," said one.

During the '50s and early '60s, when there was little love lost between Catholics and Protestants, he supported providing certain benefits, such as textbooks and transportation to parochial school students. Then in 1959, he assailed the American Catholic hierarchy for its opposition to using any foreign aid funds to promote birth control in overpopulated Third World countries.

He responded sympathetically and quickly as the women's liberation movement swept Union, moving to increase the numbers of women in both faculty and student body. When the students invited his vigorously feminist wife to be the commencement speaker in 1970, his retirement year, he proudly yielded the spotlight. At his lectures here last week, he quoted liberally from the works of a number of women scholars to buttress his points.

If, at 79, Bennett is slowed somewhat by arthritis, his analysis of the complex interplay of forces in today's world is as keen and precise as ever. Although he is not an absolute pacifist, he has lately renewed with more intensity his condemnation of nuclear warfare, which he has been voicing in his preachings and writings for more than two decades.

The reason for his increased alarm, he told audiences here, is his concern that the Reagan administration might consider nuclear war to be winnable, and therefore thinkable.

"This is a situation of utmost gravity for American Christians and for all their neighbors," Bennett said. He called for an alliance in the churches between the traditional pacifists and "those who have supported the policy of nuclear deterrence . . . .

"The possession of such murderous weapons to deter is only tolerable if it is combined with the sincere quest for arms control and arms reduction as the highest priority and if provocative and reckless foreign policies are avoided. Neither of those conditions are now being met by our government."

Bennett said he was "not suggesting that the Soviet Union does not want to throw its weight around and to seek centers of possible support for its interest as a great power, but part of this tendency arises from its fear of American power guided by more than 60 years of American hostility and now, too, closely related to the nearest enemy, China."

In the years ahead, said Bennett, "if the American churches are to be true to their calling they will see it to be their mission to seek to protect humanity against nuclear war. This will call for fresh thinking about their relations with government and new approaches both toward the nations believed to be our adversary and to the many people who fear that what they see as American recklessness may cause them to be helpless victims in the cross fire between the two nuclear giants."

Despite the grave issues with which he deals, Bennett finds hope in the long view of history. He offered as an antidote to despair over the current scene "a list of 17 distortions of Christian faith and ethics," attitudes or practices once defended by the church, but since condemned by it.

Examples, he said, included slavery, the persecution of heretics, holy wars, anti-Judaism, white racism, patriarchalism and cruel treatment of homosexuals -- the last three being in the category of distortions "that are on the moral defensive as never before . . . It is part of our inheritance today that many things which have been defended on Christian grounds are now strongly opposed on Christian grounds."

Bennett's lectures here were characteristically laden with Biblical references, not as proof-texts but employed in searching the long sweep of Judeo-Christian history, revelation and experience as a guide to confronting the problems of today.

Bennett's commitment to ecumenical Christianity remains as strong today as it was during the '50s and '60s when he was integrally involved in the National and World Councils of Churches, drafting for the latter many of its statements and position papers that have continued to guide the Council in its global efforts to achieve social justice. His commitment to ecumenism, he explained, is not to a churchly fad nor particular institution but to "a source of inspiration and correction for our churches . . . .

"Being a part of an international and ecumenical community, our churches can expect to be continually challenged by churches in other nations because they feel the effects of our government's economic policies and our political power."

"Church life will be different because our churches live in continual conversation with churches in other countries, with Christian people and their non-Christian neighbors who live in situations which are extremely different from our own," he said. "This will make life in our churches richer though often more disturbed by questions that many of us would prefer not to have anyone raise."

The antinuclear protests of Lutherans in Germany, the sharp complaints by Methodist bishops in Latin America against U.S. policy in El Salvador and the liberation theology of Catholics in Latin America are all ecumenical "corrections" to American perceptions of Christianity, he said.

Wayne Cowan, editor of the New York-based journal, Christianity and Crisis, founded by Bennett and Niebuhr some 30 years ago, credits some of Bennett's acuity to his worldwide contacts. "In his travels, he was always responsive to the worldwide church," Cowan said.

But the benefit worked both ways, said Cowan who recalled that in the '50s, Bennett went to Japan to give a series of lectures relating the church to society "that had such an impact on the Japanese church that they spoke of 'pre-Bennett' and 'post-Bennett' periods."

Cowan, who knows Bennett both as a colleague at Christianity and Crisis as well as a professor at Union, characterized him as "extremely humble and self-effacing, though he could hold his own in an argument; not only gentlemanly but courteous and always extremely charitable to all those who opposed him."

Bennett is a man who treats the language with respect and uses words with utmost precision. "He is always very, very careful; when he makes a statement, he qualifies it with great care," the editor observed.

For Cowan the archetypical Bennett is reflected in a comment during a course on systematic theology during Cowan's first year at the seminary. "He told us," recalls Cowan, " 'In theology, you can never make a statement that can never be qualified,' and then he paused a moment and added, 'Well, almost never.' "