Why did the pharaoh order the killing of male Hebrew babies but not female ones? Hebrew girls married Egyptians, dissolved among them, and were not a threat to the Egyptian community. Hebrews were close to the majority in Egypt (Exodus 1:9). Either recognize the Egyptians as matrilineal Jews (because they descended from some Jewish women), or accept that Jewishness is not matrilineal. For ages, Jewish girls who married Gentiles lost their Jewish identification.

Historically, Jewishness was patrilineal because membership in a clan was determined by one’s father’s lineage. Automatically, someone with a Jewish mother and a pagan father belonged to no clan, and fell out from Jewish society. That’s why Rashi comments that a son of a Jewish woman from Dan tribe and an Egyptian father (Lev24:10) had to convert to Judaism and had no part among Dan, as it is written: “Each man at his flag with the signs of his father’s house [not mother’s] (Num2:2). This episode had happened already when the Torah was given, and therefore patrilineal Jewishness is consistent with the Torah.

Another episode in Ezra is cited in support of matrilineal Jewishness: upon returning from Babylon, Ezra enjoined all Jewish men to send away their foreign wives. The case, however, is different: those families were extremely assimilated; they were pagan. Such way of life is apostasy per se, whether with a non-Jewish spouse or not. The children of such marriages were pronounced pagans not because of their pagan mothers, but because of their families’ (fathers included) pagan way of life. When a foreign woman attaches herself to the people of Israel, that’s it, her children are Jewish. To demand her proper conversion is nonsense: she would not acquire a metaphysical Jewish soul upon emerging from a ritual bath (compare with baptism). She can only live among Jews, and her children would acquire Jewish souls upon birth.

The matrilineal rule, interpreted literally, allows huge numbers of modern Jewish women to marry Gentiles without reservation on the presumption that whoever the father is, their children remain properly Jewish even though no Jewish marriage ceremony took place and the resulting family is a Christmas-Hanukkah one.

The Torah only prohibits intermarriage with several especially hostile nations, which should be exterminated (Deut7:2-3). Kiddushin 68b expands the prohibition to all pagans, but that view runs against the practice. Moses had a Midianite wife, and David’s grandmother was pagan. The quasi-forcible conversion of the Edomites shows that Jewishness wasn’t understood as an ethnic trait—anyone could join. Maimonides wrote in the Guide for the Perplexed that it is enough for women to know that God is One and He has created the world. There is no room for exams on Shulhan Aruch, which only a small proportion of Jewish women know, anyway.

The current situation, however, is different. Anti-Semitism and persecutions of Jews demand a high degree of commitment from the converts; the current tolerance misguides many proselytes who believe that conversion is safe. Israel provides a major economic attraction to residents of undeveloped countries, who marry Jews for the economic benefits. Most importantly, two millennia of thorough learning and struggle for survival exerted an evolutionary effect on Jews, who are demonstrably smarter than average humans. Intermarriage dilutes that genetic advantage.

Our first choice is to minimize intermarriage. If, however, it happens, we should welcome the Gentile spouses to become Jews, and certainly not refuse Jewishness to their children.

Rabbinical style conversion is not essential according to the Torah. The act of leading a Jewish life and the commitment demonstrated by marrying a Jew suffice to attach a proselyte to the Jewish people. That’s in theory. In practice, people always surround important events with rites. They dress well before going to a birthday party and invite hundreds of friends to marriage celebrations. Initiatory periods are also common: most countries require at least five years of unblemished residence before awarding citizenship. Many rabbinic rituals, a few years’ conversion period, and the requirement of leading a distinctively Jewish life—such as minute Shabbat observance—are sensible. Progressive Jews could reject the conversion rules and ceremonies, and appeal to the one-phrase conversion of Ruth, but their children could grow more traditionalist and sensitive to public opinion, and might doubt their Jewishness because their mother did not formally convert. Judaism is a communal religion, and it makes sense to respect commonly accepted traditions of conversion. Arguing against it and changing the details is fine, but as long as the tradition exists, adherence to it provides a uniform definition of Jewishness and precludes disappointment for the children of mixed marriages when they learn that they are not proper Jews in the majority opinion.

Sages realized that less-than-righteous Jews, and even apostates, retain the transcendent substance of Judaism, which is inherent. Judaism is the ultimate earthly aim of Jews, but Jewishness doesn’t equal Judaism. Jewishness, like any nationality, is a matter of blood. Americans see their national fulfillment in shining liberty upon other people; Russians see it in the Orthodox idea. Jews, likewise, exist for the sake of Judaism. A bad husband is a husband nonetheless, and an atheist Jew or the offspring of a mixed marriage is a Jew nonetheless.

If a Jew married Gentile, fast-track conversion is preferable to a two-to-four-year giyur which the Gentile spouse would not undertake. Historically, conversion has been on-the-spot. Losing huge numbers of Jewish hereditary lines through the innovative burden of long conversion is no different from murder. Attract Gentile spouses for giyur even with material incentives; they might not be good believers (Rebekkah who hid the idols wasn’t one, either), but their children could remain Jewish.

Rabbis spoke of an additional soul of Jews compared to Gentiles. Short of extravagant explanations, Gentile converts lack that soul. Ruth didn’t imagine becoming a Jew; she declared that she would join the Jews and worship God. Converts join the Jewish people but don’t become Jews. Their children acquire the Jewish soul at birth and are proper Jews.