TO WATCH Bishop Walter Sullivan of Richmond explaining the Catholic bishops' pastoral letter on nuclear war in the parish hall of the Blessed Sacrament Church is to realize how far the bishops have come -- and how far they still have to go in transforming the American church into a "peace church."

The bishop's presentation of the study and prayer that had resulted in a final draft which he said proudly put them in direct confrontation with the U.S. government, was received with warm applause from the audience of some 200 mostly older people. But the question period demonstrated vividly that for some of the faithful the new gospel of peace is as unnerving as the abandonment of Latin as the language of the mass.

Blessed Sacrament Church, which is located at Chevy Chase Circle among pleasant old homes, is a kind and open-minded parish, given to endless good works: its parishoners feed the hungry, visit the sick, care for the elderly and read to the blind. At the Pentecost folk mass, there was aerobic dancing. The Committee for Social Justice invited Bishop Sullivan to come back to the parish of his boyhood to spread the word that the church has changed.

The bishop, a tall, loose-limbed man with a long mobile face, took the microphone off its stand and held it in his hand, like a night- club entertainer.

He seemed prepared for the questions which proved that his feeling of being "thrilled" with the bishops' work is not universally shared. Since he began evangelizing about the letter, he has traveled up and down the East Coast, encountering rage, confusion and bewilderment from Catholics who feel the ground shaking under their feet as they encounter a doctrine so alien to the simple, superpatriotism of their youth.

"Jesus Christ," the bishop observed, "said you should love your enemy, not deter him."

Some take refuge in the assurance from one of the few anti-pastoral bishops, Archbishop Philip Hannan of New Orleans, that the letter is not "morally binding." One man rather edgily inquired what effect the pastoral could have when Catholics were not obliged to believe it, as Hannan has noted.

Bishop Sullivan knows his material is explosive enough, so he adopts a laid-back, disarming manner. This is another strangeness for people who were brought up with authoritarian thunderers as their shepherds.

"Well obviously, it had no effect on Archbishop Hannan," he chuckled and let it go.

The friendliest query he got was from a man who asked what influence the Vatican had had on the the letter. The bishop used the occasion to advance another heretical suggestion -- that the bishops of another day had been wrong. He noted that after Hiroshima the European bishops asked the Americans to join them in a condemnation of the atom bomb, an idea promptly shot down by Cardinal Spellman, the apostle of getting along with government.

But it was obvious that the heart of the problem is the bishops' implicit declaration that in order to achieve peace, we must deal with the Russians as human beings. Many in the parish hall had heard for most of their lives from the pulpit that the godless, atheistic Russians were the source of all evil in the world -- in other words, the philosophy of Ronald Reagan.

"How can we trust the Russians," asked a baby-faced young man. "They have broken every treaty,"

"The thing I remember about SALT II," the bishop rejoined mildly, "is that we did not ratify it. They did."

He was asked by an older man how the bishops had failed to notice the Soviet buildup -- that "the only place we are superior is in our helicopters?"

The bishop said that the Scowcfort Commission had said there was "no window of vulnerability," for which he was glad, but added that being vulnerable hadn't bothere him.

The young man came back again to the gnawing question of "trusting the Russians."

The bishop asked provocatively, "How can the Russians trust us?"

There were gasps, and one man burst out, "That's unfair. Look at the Western Europe and Eastern Europe."

The bishop reminded them that the United States had dropped the atomic bomb. In the hope of defusing the situation, he cited a familiar god, Fulton Sheen, who called it "the worst evil ever done in our world."

A man of World War II vintage leaped up in indignation. "The only reason we did was because we had it, and the others didn't. It was good that we did and got it over with. If we hadn't, we would have lost a million men invading Japan."

Yes, I've heard that, the bishop said, "I always thought there was a lot of propaganda in it."

They were "just getting warmed up" when he closed the meeting promptly at 9:30. He did it by reading a passage from the pastoral about the need to train people for peace as they are now trained for war. It was a gentle reminder that he is not the only one in a Roman collar who is preaching revolution.