The day after he was nominated for the highest post in his church, the Episcopal bishop of Washington was arrested for protesting South Africa's racial policy, along with 77 of his flock.

The next day, the Rt. Rev. John T. Walker was back in his comfortable office on Mount St. Albans, discussing the basics of Christian belief with half a dozen lively youngsters from the confirmation class of a fashionable Northwest parish.

This spanning of the two poles, faith and action, symbolizes the ministry of the man who, come September, could be the first black to head an institution that epitomizes White Anglo-Saxon Protestantism in this country, the Episcopal Church.

Walker had sent a letter to his priests, well before his nomination as one of four candidates to become presiding bishop was announced, inviting any who were so inclined to join him in the apartheid protest.

"I never expected as big a crowd," Walker said of the more than 300 who turned out. "It pleased me because, you know, Episcopalians are not marchers. They want these things left in the hands of the government,"

During the 2 1/2-hour wait to complete the processing at police headquarters, Walker found himself not altogether a stranger. He served for a number of years on the Police Chief's Citizen's Advisory Board, and took some ribbing last week from officers he'd met then. Several came up to him and observed, " 'Well, bishop, we've never seen you in this role before,' " some of them commented, he said.

If Walker was surprised by the numbers who responded to his invitation to join the protest, the Rev. Henry Bruel of St. Thomas parish was not. "When John Walker asks us to come out, we come."

Walker appears to enjoy enormous support in his diocese. He was made diocesan bishop in 1977, a year when traditionalist parishes around the country were splitting from the Episcopal Church because of the ordination of women priests and the use of a new prayer book.

But there have been no defections, no church splits in the geographically diverse Washington diocese, which stretches from the suburbs of Montgomery County, through Washington's inner city to the rural parishes of St. Mary's.

"He's been a bishop of the entire diocese," said the Rev. John Harper of St. John's Church at Lafayette Square. Harper likes that the bishop "has a presence in the city of Washington, in terms of the establishment, the social community, the government . . . . He's well liked by the clergy and the lay people."

In the Episcopal Church, the power of the presiding bishop comes more from whatever charisma and influence he has than from constitutional assignment. Unlike Roman Catholicism, Episcopal church government is decentralized and democratic.

The presiding bishop, titular head of the church, also presides over the House of Bishops and the church's executive committee and helps consecrate new bishops. But he may not intervene in the affairs of individual dioceses.

Nevertheless the once-in-12-year-election generates churchwide excitement, even though there is supposed to be no campaigning for the office. "The tradition of the church is that the persons nominated stand outside any campaign process," Walker said.

The next presiding bishop, Walker believes, will have to deal with "a lot of problems that will confront the church in the next 10 years." The issue of peace, for instance, is "one that can split the church. There are many Episcopalians who support the policies of the present administration, while others believe there should be a nuclear freeze."

Another item is "the whole racial thing" as it affects blacks and Hispanics. "There are too many clergy overall" in the church, "but too few black clergy," and in the West and Southwest, too few Hispanics.

Though the church has agreed on ordaining women, "There are now so many women priests who are not finding the kinds of jobs they are capable of," he said, while there is a surplus of male priests.

The Episcopal Church has lost 500,000 members since its peak in 1966. Nearly 60 percent of the church's 2.8 million members nationwide were not raised as Episcopalians but joined as adults.

"Reconciliation is what my ministry is about," he said, "but reconciliation that always ends up with justice, and that makes some people very angry."

Walker, who is a member of the Club of Rome, the futurist society, wants his church to do more long-range study and planning. "The church needs to be thinking about what the form of the church will be like in the year 2000, what the population distribution will be, whether we'll need a lot of small churches or a few large ones," he said.

Born in 1925 in Barnesville, Ga., where his great-grandfather had founded the Barnesville African Methodist Episcopal Church, Walker moved with his parents to Detroit when he was 2, and he was educated there.

He switched to the Episcopal Church after becoming involved with an activist youth group and getting to know the dean of Detroit's Episcopal cathedral.

He was the first black man to graduate from Virginia Theological Seminary in Alexandria. After ordination, he became rector of an integrated parish in Detroit for three years, then went to St. Paul's School in Concord, N.H.

He taught in a church seminary in Uganda for a year and came here in 1966 as canon of the Washington Cathedral. He was elected a suffragan, or assistant, bishop in 1971, and five years later was overwhelmingly chosen to succeed Bishop William F. Creighton as head of the Washington diocese.

With his wife, Rosa Maria, whom he met in 1961 while leading a summer training program in Costa Rica, he likes to putter in the garden of their Cleveland Park home and read and write poetry.

If he is elected next September, he will have to leave Washington for New York, where the church's national offices are located. "It would be sad to lose him," said the Rev. Janice Nunnally, pastoral counselor at Columbia Hospital for Women.