A CERTAIN PEOPLE; American Jews and Their Lives Today. By Charles E. Silberman. Summit. 458 pp. $19.95.

IN JULY 1883, the first graduating class of Hebrew Union College -- America's pioneer rabbinical training school -- gathered at Cincinnati's Highland House for a banquet. On the menu that evening were littleneck clams, soft-shell crabs, oysters and shrimp -- all foods prohibited under Jewish dietary laws. The shellfish were served at the express wish of the wealthy Jewish businessmen who underwrote the banquet -- eager for social acceptance, desperate to prove how American they were.

Nearly a century later, Charles Silberman, his wife and four sons were invited to meet Vice-President Hubert Humphrey in his ceremonial office at the Capitol, cluttered with trophies from a lifetime of public service. While Silberman and his wife chatted with the vice- president in one corner of the cavernous chamber, his third son shouted from the other end, "Hey, Dad, come look at the Torah in the showcase!"

In A Certain People, Silberman tells the story of American Jews' passage from the ethos of self-effacement, the craving for assimilation and even invisibility, to the ethos of self-assertion, the cool self-confidence which proclaims Jewish identity in a pluralistic society. It is a resonant tale, in which sound the great themes of our national experience.

For Silberman is grappling here with issues which transcend narrow Jewish interests -- the tension in American life between the universal and the particular, the cosmopolitan and the parochial, the common weal and the special interest -- themes which are equally salient among Irish Catholics, Italian-Americans, blacks, Indians or Hispanics.

Unfortunately, in the book's middle reaches, we almost lose track of these major questions. In his eagerness to show his readers how far American Jews have come, Silberman seems at times to be pandering to a kind of Jewish babbittry. We get breathless portraits of Irving S. Shapiro, the chief executive officer of E.I. du Pont de Nemours and Company, and Gerald Greenwald, Chrysler's vice- chairman; a wide-eyed view of America's real- estate industry; long rosters of Jews who have made it into Congress, the White House staff, and the upper ranks of law, medicine, academia and journalism.

We get endless statistics about how wealthy, well-educated -- even psychologically stable! -- American Jews are. We are told that Jewish professors are "more likely to publish articles in scholarly journals than their non- Jewish peers," that "30 percent of American Nobel laureates in science have been Jews," that "23 percent of the people on the Forbes 1984 list of the 400 richest Americans were Jews" and that when Charles Kadushin compiled a list of America's 21 most eminent intellectuals, 15 were Jews. We get a host of self-serving statements, topped by Marvin Kalb's answer as to why so many Jews had percolated into high-paying journalistic jobs: "You can't say no to quality."

But Silberman is pursuing a larger issue after all. "Since the end of World War II," he writes, "and at an accelerating pace since 1960, the United States has become an increasingly open, pluralistic and meritocratic society, in which position depends on what one can do rather than on who one is; and Jews are far from being the only beneficiaries."

He is especially concerned with the flip side of that proposition -- the threat which an open society poses to ethnic identity. Citing editor Leonard Fine ("It is seduction, not rape, that (American Jews) fear the most,"), he addresses himself to repeated warnings that a combination of relentless assimilation, a declining birth rate and intermarriage have made American Jews an "endangered species."

SIX YEARS of research have convinced Silberman that "Judaism is not about to disappear in the United States." Quite the contrary, he concludes, "a major renewal of Jewish religious and cultural life is now under way."

What has happened, he contends, is that just as the second generation of American Jews tried to shuck off their fathers' identity in order to get ahead, so a newly-secure third generation -- and often the fourth and fifth -- are recapturing that Jewishness.

Silberman cites the dramatic example of the New York writer, Paul Cowan, whose father, a CBS executive, had changed his name from Cohen, and whose mother had been raised a Christian Scientist, a faith to which her parents converted in 1910. Their son was sent to Choate, an Episcopalian prep school with compulsory daily chapel, and then on to Harvard. Now, in middle-age, Paul Cowan has embraced an intense communal Judaism and his wife, Rachel, born a New England Protestant, has recently converted to Judaism as well.

Silberman argues that not only are there many Paul Cowans throughout the country, but there are many Rachel Cowans too. "A significant minority of born-Gentile spouses -- approximately 20 percent -- convert to Judaism," he says. Finally, after closely analyzing several alarming reports about declining birth rates, Silberman concludes that "there is no convincing evidence to suggest any significant decline in (Jewish) fertility."

If American Jews are no longer "guests" in someone else's country but part of the "host" people, if virulent anti-Semitism has largely disappeared (except, he argues, among some of the black middle class), if American Jews are far more secure in their professions, their homes and their futures than ever before, what stance should they take toward this land they have inherited?

Silberman flatly rejects the prescriptions of conservative Jews like Irving Kristol and Lucy Dawidowicz that Jewish -- and Israeli -- interests now call for an allegiance to Ronald Reagan's brand of Republicanism. Noting that a majority of American Jews voted for Walter Mondale last fall, he believes that they decided that "a country governed by a Democratic administration was, indeed, more likely to be hospitable to Jews than one governed by Republicans," because "Jews are safe only in a society acceptant of a wide range of attitudes and behaviors, as well as a diversity of religious and ethnic groups."

After the prosaic middle sections of the book, Silberman's peroration at the end is genuinely moving. The choice confronting his fellow Jews is clear, he writes: "whether, through fear of strangers we live like weaklings behind walls of our own construction or whether we have the courage to live like mighty warriors in this great open place we call the United States."