In 1391, the escalating persecution suffered by Iberian Jewry led to mass conversions, which continued as the Jews’ situation worsened. The presence of large populations of former Jews, and the return of some of their descendants to Judaism in subsequent generations, challenged the way both Jews and Gentiles understood Jewish identity. David Graizbord explains how:

By . . . 1415, some half to two-thirds of Castilian and Aragonese Jews had become titular Christians. Irrespective of the sincerity [of their conversions], or lack thereof, in many if not most cases the converts and their immediate, baptized descendants still lived among, or relatively close to, Jews and had extensive social, economic, and familial relations with them. This meant that for the first time, the religion and the ethnicity of tens of thousands of people once known and still widely regarded as “Jews” were at odds: these “New Christians” were “Jewish” as concerned their social and economic relations, their ethnic culture, and their social reputation, yet their religious identity was at least theoretically identical to that of the majority population.

Of particular interest in this connection is the promulgation as early as 1436 of private and municipal statutes of “Purity of Blood.” This concept formally recast and stigmatized Jewishness as a matter of descent rather than of official religious status, much less of demonstrable belief and behavior. Equipped with this new notion of purity, “Old Christians” began to treat questions of morality and religious fealty as matters of familial heredity.

For their part, Jews acquired a correspondingly acute consciousness of their genealogy. . . . Jewish letters of introduction from the late 1300s and early 1400s, [i.e. from when mass conversion began], differ from older ones in explicitly distinguishing between “good” Jewish families—that is to say, families whose members had not converted—and families sullied by Christianization. A fateful message of the letters was that while Iberian Jews may share ethnicity, their differing fealty to God rendered them essentially separate. What now mattered for purposes of determining a Jew’s character as a Jew, was not only the quality of his or her behavior as an observer of mitzvot, but the caliber of his or her yiḥus [lineage].

Two extraordinary events demonstrate something important about Israel’s most fervent adversaries. One was a speech given at something called The People’s Forum (funded generously by Goldman Sachs), which stated, “When the state of Israel is finally destroyed and erased from history, that will be the single most important blow we can give to destroying capitalism and imperialism.”

The suggestion that this tiny state is the linchpin of a global, centuries-old phenomenon like capitalism goes well beyond anything resembling rational criticism. Even if Israel were guilty of genocide, apartheid, and oppression—which of course it is not—it would not follow that its destruction would help end capitalism or imperialism.

The other was an anti-Israel protest that took place in front of New York City’s Memorial Sloan Kettering Cancer Center, deemed “complicit” in Israel’s evils. At organizers’ urging, participants shouted their slogans at kids in the cancer ward, who were watching from the windows. Given Hamas’s indifference toward the lives of Gazan children, such callousness toward non-Palestinian children from Hamas’s Western allies shouldn’t be surprising. The protest—like the abovementioned speech—deliberately conveyed the message that Israel is the ultimate evil and its destruction the ultimate good, cancer patients be damned.

The fact that Israel’s adversaries are almost comically perverse does not mean that they can be dismissed. If its allies fail to understand the obsessive and irrational hatred that it faces, they cannot effectively help it defend itself.