Robert Lucas, an 18-year-old Presbyterian from Marion, Ind., spent the last three weeks at American University talking about his faith with 59 other high school graduates, most of whom were not Presbyterians.

"Coming from Indiana, it's a little bit of a cultural shock but also fascinating," said the Harvard University-bound Lucas. "I'd never had a chance to sit with a rabbi and talk in an environment where it was acceptable to ask questions."

Aaron Jenkins, a Banneker High School graduate who worships at New Solid Rock, a small Pentecostal church in Northwest Washington, was also among the teenagers exploring religious diversity. He came away with one big lesson.

"Question everything, even things you hold dear," said Jenkins, 18, who is headed for Williams College in Williamstown, Mass., this fall. "In order to understand it, you have to question it."

Lucas and Jenkins were participants in "E Pluribus Unum," or "Out of Many, One," a program that brings together 20 Protestant, 20 Jewish and 20 Catholic students for a kind of spiritual reconnaissance: Go examine your religious roots to see what they say about peace and social justice, they are told, and then regroup to see what you all have in common.

This year's session, which ended Saturday, was the third for the program founded by Rabbi Sidney Schwarz, director of the Washington Institute for Jewish Leadership and Values in Rockville. The Institute runs the program in cooperation with the National Federation for Catholic Youth Ministry Inc. and the National Council of the Churches of Christ, U.S.A.

Schwarz, who has long been active in local interfaith activities, said the program works on three premises: that people of faith need to know their own religion well before they can develop a diverse community; that each faith is "only a piece of a larger truth"; and that "the true test of religion is whether it's addressing issues of peace and justice."

Every morning, the Jewish, Catholic and Protestant students attended separate "Faith Alike" classes with teachers of their own religion to explore its moral perspective on human rights, poverty and the environment.

The Protestant group, which included Pentecostals, Episcopalians, Methodists, a Mennonite, Baptists and others, posted "Ground Rules" in its classroom. These included "Openmindedness," "Present and Awake," "Put God (Higher Power) First," "Respectful and Loving Debate--Not Argument" and "Seek Common Ground."

The rest of the day was filled with such interfaith activities as volunteering at local shelters and listening to speakers on a variety of subjects, including the death penalty, hunger and what it's like to work for AmeriCorps. The students also examined how art can be used to express social concerns and spent a day studying political advocacy on Capitol Hill. There were student-run interfaith worship services and opportunities for students of one faith to attend services of others.

Julia Bloom, a Jewish resident of Palm Beach Gardens, Fla., and a freshman at Duke University this fall, said she attended her first Christian service.

"But what makes this program so different and unique," she said, "is that we are taught to recognize different injustices but also to look at practical solutions."

Each evening, the students broke into small, intense, interfaith groups to discuss what they had learned, raise fundamental questions about life and seek common ground. Difficult moments arose, for example, when talking about abortion.

"There were differences and conflict because we do have different beliefs," said Martha Swaney, 18, an Episcopalian from Arlington, Tex. "But overall, we did mesh much more than I could have imagined or expected."

Some students complained that too many of the program's speakers were liberals, with conservative views given short shrift.

"When it focused on religion, it was absolutely wonderful," said Andrew Reinicke, a Catholic from San Diego, who will study international relations at Grossmont College in El Cajon, Calif. "But when it focused on political issues, I found it upsetting [because] we had speakers who were very one-sided."

The students are headed for colleges with diverse communities. But James P. Keen, an authority on intensive learning environments, said most would not have the experience provided by "E Pluribus Unum."

When researching his book, "Common Fire: Lives of Commitment in a Complex World," Keen said he found that people committed to social justice had all had "an enlarging encounter that had taken them across the threshold of differences in a way that changed them profoundly." He cited the experience of Peace Corps volunteers who really got to know the people in the countries they served.

Schwarz may add Muslim students in upcoming sessions, a move endorsed by Ilham Nasser, a Muslim and counselor at Marymount College who was invited to observe this year's session. "You can't do these interfaith groups without including Muslims," she said.

CAPTION: Catholic, Protestant and Jewish students compare their religious roots at three-week American University workshop. From left are Andrew Reinicke, Natalie Newfield, Martha Swaney, Julia Bloom, Robert Lucas and Aaron Jenkins.