Feeling the need for absolution, I went to church Saturday night. In the pulpit was no home-grown American minister, but a famous African bishop, the Right Rev. Desmond Tutu, Anglican bishop of Johannesburg, Nobel laureate and antiapartheid leader.

An hour before his arrival, the vestibule of the Peoples Congregational Church on 13th Street NW was jammed to overflowing. Every square inch of the sanctuary was full. The main auditorium and balcony overflowed, and people were seated in adjacent rooms where they could see and hear the bishop on closed-circuit television.

When Bishop Tutu entered the sanctuary, the audience was singing a song. But they stopped and burst into applause at the sight of the diminutive man in a blood-red robe.

Surrounded by ministers of all denominations, male and female, black and white, Bishop Tutu walked slowly down the aisle, a kindly smile pinned upon his tired features. Sitting in the pulpit, he looked at a program that said, "Peace With Justice for South Africa: A Winter Convocation," as another clergyman prayed.

In the loft behind the pulpit, a white-robed choir stood and sang in tones so glorious that Bishop Tutu twirled around in his seat to listen. At the song's conclusion, an old man's loud and solitary "amen" came from the rear of the church.

As a young black American woman with cornrowed hair, Jean Sindab of the Washington Office on Africa, spoke passionately about the evil of apartheid, Bishop Tutu looked at her and listened intently. "Two thirds of the 1,300 blacks who have been killed in recent months are children and young people," Sindab said. "We are not here to intellectualize but to understand this reality."

Introduced as a liberator and great theologian, Bishop Tutu approached the pulpit with great ease. As a hymn called "Thank You Lord" was sung, his laserlike gaze swept the mixed-race audience.

"Thank you for your love," he began. "Thank you for your caring. What you have done shows the world that ordinary people can make a huge amount of difference," a reference to U.S. antiapartheid demonstrations. As the demonstrations have persisted, the Reagan administration has reversed its policy of constructive engagement and imposed limited economic sanctions on the Pretoria regime.

But unlike previous addresses when he lashed out with great fire at his country's white domination of its black majority, the bishop spoke this night as a preacher of the gospel. "We have," he said, "a marvelous God. He sent his son to restore human community and the brotherhood which sin destroyed."

Bishop Tutu looked so comfortable it was hard to believe that he did not belong to that black church and black denomination in the heart of Washington.

He made it clear that God requires from His people a total commitment to eradicate injustice, poverty and hunger. "God wants us to be his partner," he said. After 15 minutes, Bishop Tutu bowed his head to signal the sermon's end. The audience rose and clapped while the bishop stood, head still down, as if in prayer.

The United Church of Christ, the American Friends Service Committee and the Adelphia Foundation are sponsoring Bishop Tutu's U.S. tour, but at each church where he speaks individual ministers take the lead in planning the program. Peoples Congregational Church's Yale-trained minister, The Rev. A. Knighton Stanley, put together the convocation aimed in part at uniting African and Afro-American traditions.

A medley of freedom songs and traditional Negro spirituals included not only James Weldon Johnson's "Lift Every Voice and Sing," known as the black national anthem, but also the African national anthem. Bishop Tutu spiritedly sang both. When it came to singing his own national anthem, though, he sang not in English but in his native Zulu.

At the end of his sermon, Bishop Tutu left the pulpit for a few minutes to greet personally those persons who had been unable to crowd into the main sanctuary.As he returned, the congregation was singing "We Shall Overcome," and the service was beginning to take on the atmosphere of a 1960s civil rights activist meeting. Andrea I. Young, who is Stanley's wife and the daughter of Atlanta Mayor Andrew Young, even reminded Bishop Tutu that an international outcry like the one being raised against Pretoria helped rid the United States of segregation in the 1960s.

As the evening drew to a close, the audience gave Bishop Tutu and his wife Leah an estimated $20,000 for their work in South Africa. And Bishop Tutu ended the evening as he had begun it. "We are so grateful for your support," he told the audience. "We are a family."

Then it was time to leave, and Bishop Tutu, sounding for all the world like a southern preacher, thanked the choir for its music. As he walked down the aisle, in his blood-red robe, he held his Bible firmly and lifted his right hand to gently wave goodbye. For a couple of hours, the witty raconteur, the voice crying out against South African racism and oppression, the man who faced death threats and a possible apocalypse in his own land, had been simply a preacher.