For more than 30 years, Arthur James Armstrong fought the good fight of a social activist clergyman.

He crusaded against the war in Vietnam and for civil rights at home. He championed the Third World against exploitation by multinational firms. He battled manufacturers of infant formula whose marketing practices in developing countries, the activists charged, killed babies by the hundreds.

Predictably, he was bitterly criticized by many, but his ministry was affirmed, and he was elected a bishop, then president of the National Council of Churches. In 1982, a national news magazine named him the most influential religious leader in the country.

Then, a little more than a year ago, he suddenly resigned as president of the National Council of Churches and as United Methodist Bishop of Indiana and signed himself into the Menninger Clinic in Kansas.

A public statement vaguely blamed "an exhaustive and inhuman work schedule" that left him "physically and emotionally depleted" for having "failed my family and other loved ones." There were ugly rumors -- particularly when his wife, Phyllis, filed a divorce petition that was later withdrawn -- that not all of the Commandments had been observed.

A year later, layman Jim Armstrong -- he has turned in his ministerial credentials -- the one-time critic of multinational corporations, is a senior consultant for the International Business-Government Counselors, Inc. The Washington-based firm describes itself as "an international government relations counseling firm providing intelligence, research, counseling and problem-solving assistance."

The company whose problems Armstrong spends most of his time trying to solve is Nestle, the Swiss corporation whose marketing practices triggered the worldwide infant formula war more than a decade ago.

Armstrong will not discuss the more personal aspects of last year's abrupt turn in his life. "The less said about that the better. Enough people have been hurt already," he said.

He and his wife are living together in Alexandria. He is writing a book, which Harper and Row will publish next year.

No, he insists, it will not be a shocker. "I delve into the past to the degree it will help the reader understand present reality."

Subject closed.

But he talks easily about his new work, which he likes to characterize as "building bridges."

Yes, there was a time when he led the churches' battle against Nestle, he recalled. In 1979, the United Methodist Church asked him to go to Switzerland to meet with executive leadership of Nestle. The meeting accomplished nothing.

"It was a stalemate at its most vitriolic," he recalled. The next year, at the United Methodist quadrennial General Conference, he urged a boycott of the company's products.

The years of hammering away at the infant formula problem by a host of church groups, unions, other activists and even congressional investigating committees bore fruit in 1981 in the form of a World Health Organization voluntary code designed to regulate the marketing of breast milk substitutes.

Meanwhile, the top Nestle officials with whom Armstrong had tangled in 1979 were replaced by new executives, for whom "I came to have tremendous respect," during subsequent negotiations with church groups over Nestle's acceptance of the marketing code, he said.

The respect was apparently mutual. When Armstrong cut his church ties last year, he said, "Almost immediately, Nestle was in touch with me, asking would I come to work with them."

He didn't accept at once. "When you are in a sense recharting the course of your life, you need to consider a variety of options." After a five-month stint as a consultant for Broward Community College in Florida earlier this year, he moved into his present position.

For a couple of decades, the World and National Councils of Churches, in which Armstrong played leading roles, and their generally liberal member churches have tended to find themselves on opposite sides of the table from big business. Controversial issues have ranged from the manufacture of weapons to investments in South Africa to workers' rights.

Armstrong hopes that he can use his church contacts in his new position to help resolve some of the conflicts. "There are good people on each side that are concerned about these issues," he said. "No one has all the virtues."

No, he says to the suggestion that he has made an about face, "it's not a 180 degree turn." Mediating between the big corporation and the social activists "is simply an effort to write into concrete reality that which I talked about in my ecclesiastical garb for a long time."

Not all of his former church colleagues are convinced, he concedes. One of his current projects is an attempt to resolve the boycott of Campbell Soup Co. by some church and labor groups over wages paid farm workers.

He recalls a recent incident in a meeting between company heads and church leaders. "A leader from the Church of the Brethren asked me, 'How can you shift from adversary to advocate?'

"I said I'm not an advocate. . . . At one time I was an adversary, but I consider myself an ally of both sides now." He acknowledges that others of his former church associates have probably had similar doubts.

Church leaders involved with issues relating to major corporations are reluctant to pass judgment -- for the record -- on Armstrong's efforts.

"He plays an important role in building bridges of communication between the church and the corporate sector," said Timothy Smith, executive directer of the Interfaith Center on Corporate Responsibility, which coordinates stockholder challenges of major corporations based on $10 billion worth of pooled church holdings.

"At the same time, there is an important role for aggressive church advocacy to press the cause of corporate responsibility."

Armstrong is aware, he says, of the possibility of being "used."

"But I've warned people that my name is not for hire, and that has been accepted."

Armstrong said he is not too worried by criticism.

"What matters is that I've got to live with myself. If I can do that -- criticism I can endure just as I have in times past when different sort of critics have converged on me."

Although he no longer holds credentials as a United Methodist clergyman, Armstrong has not ruled out returning to the ministry. He surrendered his credentials last spring, he said, because "at the time I had been invited by the United Church of Christ" to take a Florida job, and according to Methodist rules, returning ordination documents is the first step in switching denominations.

The UCC job is still open "if things don't work out here," he said