That question has haunted me since I read an article in the August issue of Moment (not available online) about Congregacion Hebrea de Baja California in Tijuana, led by a former Methodist minister who converted to Judaism, Carlos Salas Diaz. Almost all of the congregants are Mexican Catholics who converted to Judaism, according to this article from the L.A. Jewish Journal. The article gives the flavor of the subterranean Jewish presence deep in Mexico:

The synagogue hopes to open its doors to a large influx of Indian Jews from Venta Prieta in the state of Hidalgo near Mexico City. Salas explains that they are a small township of more than 6,000 people (some accounts have pegged the number at a fraction of that number, perhaps only 200) who claim to be descendants of the Marranos or Crypto Jews who fled the Spanish Inquisition. "Everybody thought they were Indians because they didn’t understand what they were saying. In fact they were speaking Hebrew," Salas said. Some have shown an interest in migrating to Tijuana since the largely Orthodox congregations of Mexico City question their Jewishness and are reluctant to accept them, Salas said. "We have welcomed them without questions."

The Moment article struck me when one source, perhaps Salas Diaz, estimated that 30 million Mexicans could have a Jewish background, through conversos who fled to Mexico to escape the Spanish Inquisition.

Whether that's possible or sheer fantasy, it made me think. What if it's true? What if the Mexicans coming to the U.S. gravitate to shuls rather than churches, and declare themselves our long-lost landsmen? Certainly the idea of Lost Tribes in Mexico has some supporters, such as the late Texas rabbi Samuel Lerer. The article, "Coming Out of the Judaic Closet in San Antonio," gives another perspective from a woman converted by Rabbi Lerer.

I don't have an answer, but the idea of millions of Mexican Jews raises tantalizing questions. Good sources of more information are the Society for Crypto Judaic Studies and Kulanu, which tracks hidden Jewish communities worldwide and has an outstanding links page.