Why is it that a child of a mixed marriage is considered to be a Jew only when the mother is Jewish?

The answer is tradition -- tradition that has become frozen into Jewish law.

An organization of Reform rabbis meeting here this week came close to overturning that law and replacing it with a plan to give fathers equal rights with mothers in imparting Jewishness to their offspring.

After hours of debating a carefully framed resolution, which a committee of scholars had worked two years to perfect, the Central Conference of Reform Rabbis voted 179 to 168 to send the resolution back to committee for further clarification.

The question of who is a Jew has been an issue throughout history. From ancient times tradition has held that a child born of mixed parentage is a Jew only if the mother is Jewish. If the father is Jewish and the mother is not, then according to the law the child must undergo formal conversion to be considered Jewish.

A number of factors have given the debate over Jewish identity added urgency in recent years:

* Mixed marriages are increasing. Studies indicate that about 35 percent of all Jews who marry choose a spouse outside their faith.

* Declining birth rates combined with the memory of the Holocaust have increased the anxieties among Jewish leaders over the perpetuation of the Jewish people.

* In light of current movements for women's equality, in whch Reform Jews have played leading roles, a religious stricture that so clearly discriminates against men has become increasingly nettlesome.

Reform Judaism, with roughly 1.2 million adherents, is the most liberal of the three branches of Judaism in this country. Three years ago, Rabbi Alexander Schindler, who heads its lay organization, called for a change in the tradition of imparting Jewishness only through the mother, and the rabbis established a scholarly committee to study the question. The result was the one-sentence proposal placed before 600 rabbis attending the annual meeting here this week.

"When only one of the parents is Jewish," the proposal read, "the Jewishness of the child is derivable from the Jewish parent and is expressed by participation in Jewish life."

The resolution was purposely stated in general terms, explained Rabbi Herman Schaalman of Chicago, president of the Conference and head of the committee that prepared the resolution. "The operative word is 'derivable,' " he explained. "To make it permissive, not prescriptive . . . that under the proper circumstances, this child may be declared Jewish."

Schaalman acknowledged in a discussion of the resolution that "there is a wing of Orthodoxy that will totally excoriate us . . . who will take some action to disqualify us as Jews if we take this action."

An even greater area of concern centers on the relationship of Reform Jews to Israel. "The flashpoint is the Law of Return," explained Rabbi Joseph Glaser, executive vice president of the conference, referring to the Israeli law that guarantees to any Jew the right to settle in Israel and gives substantial concessions and privileges to Jews immigrating there.

Under that law, a Jew is defined as anyone born of a Jewish mother or who has converted to Judaism, irrespective of whether the conversion was done by an Orthodox, Conservative or Reform rabbi. This definition is under fire.

In Israel, Orthodox Judaism dominates all aspects of Jewish religious life because of a complex political situation. For years, neither of the two major political parties has been able to command a majority without alliance with two tiny Orthodox parties, whose price for cooperation is a virtual monopoly on Jewish religious life.

The Orthodox have maintained continuing pressure on the government to declare only Orthodox conversions valid in implementing the law of return. Glaser said that adoption by the Reform movement of the new definition of Jewishness could stir enough animosity in Israel that the Orthodox parties could conceivably "pick up enough votes in the Knesset to change the law of return."

In an impassioned speech in defense of the proposal, Schindler discounted this argument and pleaded with his fellow rabbis to put aside Israeli politics on this issue and "do what we believe and what is right" and approve the so-called "patrilineal descent" resolution.

In the debate over the resolution, some rabbis wanted to go beyond the committee's recommendation and make "participation in Jewish life" a condition of acknowledging the Jewishness of a child of a mixed marriage.

"There is no basis in fact or Jewish tradition for the argument that a child of a mixed marriage born by happenstance to a Jewish mother is biologically, genetically Jewish," argued Rabbi Joseph Edelheit of Michigan City, Ind. "We are not a race, though our enemies in the past and currently have tried to affix such a label."

Initially Edelheit's proposal won an easy victory. As the debate continued, however, the rabbis had second thoughts. "I haven't a clue as to what 'participation in Jewish life' means," complained one.

"Whose 'participation'?" challenged another. Are we talking about the participation of a 3-year-old?"

The resolution "says nothing about the child not being raised in another faith" another rabbi objected. "There's nothing that says one parent can't take him to church every Sunday and the other to Jewish services every Friday night."

Several members complained that the participation amendment, instead of broadening the acceptance of children of mixed marriages had in fact limited it. "How can we, in good conscience, say to anyone who wants to be a Jew: 'You are not a Jew,' " said Rabbi Sheldon Zimmerman of New York City.

After unsnarling itself parliamentarily the conference voted to rescind the "participation" amendment. Then, as midnight neared, Rabbi Arthur Lelyveld of Cleveland, a widely respected former president of the conference, gained the floor and moved to send the whole matter back to committee directing that body to "take into consideration the arguments that have been presented in this debate." His motion calling for "reconsideration and a full position paper" to be presented at next year's meeting in Los Angeles, was approved 179 to 168.