It looked like your typical holiday crowd--shorts, sun dresses, bandanas, sandals, picnic lunches and scampering children. But the 500 or so people gathered in the gym of Catholic University on Memorial Day morning had more than pleasure on their minds. Well over half of them had signed up to go to jail for nonviolent civil disobedience.

They were attending the first national conference on the rise of Christian conscience and of civil disobedience. It was sponsored by the Sojourners, an ecumenical group committed to the "biblical vision of peace and justice," and it brought together several church-based protest groups.

The opposition of those groups to various Reagan policies, particularly in Central America, has become "formidable" in the word of one expert: "We don't normally think of them as political opponents," said Langhorne A. Motley, departing assistant secretary of state for inter-American affairs, "and we don't know how to handle them."

They laid out plans to demonstrate at six offending capital sites or, as they put it, to "show a seamless garment of nonviolent resistance to policies against the sanctity of life." In their view, that covers the State Department on Central America; the White House on arms control and poverty programs; the South African Embassy on apartheid; the Soviet Embassy on Afghanistan; the Supreme Court on the death penalty, and Department of Health and Human Services on the unborn and women.

Afghanistan and abortion are the only two points on which they share a common view with the muscular Moral Majority, whose ideology and tactics are anathema to them.

"Seeing Christian feminists demonstrating against abortion will really blow the polarities of right and left," said Jim Wallis, editor of Sojourners magazine, who was arrested two years ago in the Capitol rotunda for an anti-MX action. "We object to the arrogance of the right-to-lifers, but we acknowledge that abortion is murder. And at the same time you have to work on a system where abortion will not be the only choice for a woman."

The decision to go to jail for their beliefs was, for some, fairly easy.

"I have done everything else," sighed Ann Marie Phillips, 57, a sister of the Holy Union order. "I've taught adult clases; I've written letters to Congress; I've marched. Now I feel that this is really necessary."

But for a handsome, young, red-headed man who was pacing up and down the gym floor, it was an agony. He was an environmentalist who wouldn't give his name. He envied those of his elders who, like Sister Ann Marie, had made their choice.

"When I think of the suffering of the victims, especially in Central America, I hate myself for hesitating," he said. "But I can't face the loss of control if I get arrested. I've made up my mind a dozen times, and then it melts away. I know I could go tomorrow and let the spirit move me, as they say here. But I'm not sure it will. I'm not into ritualistic, organized religion."

Members of the Central American protest group, the most popular, got their instructions from Vicki Kemper, who gave them a variety of spiritual and practical tips. They were to pause and pray en route, "to maintain the spirit of what we are doing and why." They should bring jackets that could be used as pillows in the cells. And they should fill out bail application forms, so that when relatives called to find out where they were at day's end, a Sojourners aide could say, "They're fine -- they're in jail."

The afternoon was given over to discussions of what to do if a Moral Majority member were to grab the bullhorn or if hecklers snarled, "Go back to Russia." Prayer was counseled. They could welcome sympathizers who might join them, but there was a strict ban on anti-Reagan posters -- "They are in themselves a form of violence."

The Christian peace-seekers seem undaunted, despite the growing militarism and militancy of the Reagan administration. But they are proud of Motley's tribute, and they point out that the Justice Department crackdown on the sanctuary movement has only prompted more churches to open their doors to political refugees from El Salvador and Guatemala.

But they are ready to go to jail whether they have any impact or not. They are doing it for themselves.

They were even enjoying themselves in a mild and earnest kind of way. In contrast to many people in Washington, particularly those in power, they know exactly what they believe and want no truck with compromise. Unlike the Moral Majoritarians, who believe the Lord is on their side, they make only the modest claim that they are on His.