Three leading rabbis from American Judaism's three feuding branches met for a "unity symposium" here last week, calling for unity but expressing disagreement over who is a Jew.

"This is one of those cases where the achievement was in the meeting itself, not in what was said there," said Stuart Eizenstat, a former aide to President Carter who was one of the organizers of the symposium.

"I think it was a major and potentially historic step forward to begin a dialogue to see if we can bridge a gap that is growing larger and larger."

The meeting Sunday morning was attended by about 800 persons at Beth Shalom Congregation, an Orthodox synagogue at 13th Street and Eastern Avenue NW. Similar sessions are expected soon in other cities across the country.

The most emotional issue dividing the three groups was the decision several years ago by Reform rabbis to consider the children born of Jewish fathers and non-Jewish mothers as Jews. Traditionally, only the children of Jewish mothers are considered Jews, regardless of whether their father is Jewish.

Rabbi W. Gunther Plaut of Toronto, past president of the Central Conference of American Rabbis (Reform), said the change was necessary to deal with "the overwhelming reality" of intermarriage between Jews and Christians. Its purpose, he said, was "not to dilute the Jewish people but to draw in the children of mixed marriages . . . to make them come close to us."

Rabbi Mordecai Waxman of Great Neck, N.Y., past president of the Rabbinical Assembly (Conservative), said the Reform decision was an "assault" on Jewish unity. Unless children who cannot claim matrilineal Jewish descent are formally converted, he said, many who see themselves as Jewish will not be free to marry each other with Jewish rites.

Rabbi Walter Wurzburger of Lawrence, N.Y., past president of the Rabbinical Council of America (Orthodox), said the Reform acceptance of patrilineal descent had "set us back tremendously in terms of the rhetoric of polzarization and divisiveness of the Jewish people."

Wurzburger said there are "theological differences that cannot be negotiated," but he urged that rhetoric be "deescalated."

"We should join hands in our common battle against the erosion of the will to survive as Jews," he said.

In an opening statement, Eizenstat said the greatest threat to Jewish survival today "is not as it has been for 3,000 years its external enemies, but it is internal . . . the threat of assimilation, intermarriage, declining birth rates, and the growing gulf between our different denominations."

Eizenstat is a board member of the National Jewish Center for Learning and Leadership (CLAL), which organized the symposium. It was sponsored by the Jewish Community Council of Greater Washington and three other groups.

Within two decades, Eizenstat said, America's 5.8 million Jews may be "irreparably split between two hostile groups unwilling to marry within our faith."

He quoted an estimate by CLAL's president Rabbi Irving Greenberg that in 20 years there will be between 750,000 and 1 million people "whose Jewishness is contested" because they are the children of mixed marriages, of those converted by Reform or Conservative rabbis, or of remarried parents who did not obtain a traditional Jewish divorce.

Wurzburger said this might cause "very major problems because we might have Jewish organizations headed by persons who by definition are not regarded as Jewish."