My first historical romance novel, The Flames of Prague (which is in the process of being broken into two novels), centers around the 1389 massacre of the Jews of Prague. Long story short, a poor knight–Sir Jakub–rescues a young Jewish woman–Alzbeta–from the massacre and marries her. The second book, which takes place 20 years later, finds Jakub, Alzbeta, and their six children living as crypto-Jews. When they’re discovered and denounced, they have to make some hard choices between their faith and their lives.
That may sound terribly implausible, but after writing the story, I came across a reference to a 16th century Italian story of a Christian noble and Jewish girl who fell in love. The outcome was the opposite–the Jewish girl converted to Christianity–but it shows that intermarriage/sexual mixing happened in the middle ages, too. (The fact that Jews always look like the people they live amongst also proves that intermarriage/sexual mixing has always happened).
Many errors and offensive acts occur between Christian men and Jewish women and between Christian women and Jewish men as a consequence of their living together in cities and dressing alike. In order to obviate the errors and evils that might result from this situation, we consider it proper and decree that all Jewish men and women living in our kingdom wear some sort of mark upon their heads so that all may clearly discern who is a Jew or a Jewess. (13th century Spanish law)
The church had good reason to fear mixing. While conversion to Judaism was not common, it did happen. Andreas, the Archbishop of Bari converted around 1066, as did a priest, Fr. Johannes (aka Obadiah the Proselyte) in 1070. There is also a reference (sometime before 1200) of two Welsh Cistercian monks who converted. Many more people converted and just slipped quietly away from Christian society. (As recently as the 19th century, a Quaker-turned-Jew, Warder Cresson, was declared insane by his family when he converted; he had to go to court to prove his sanity and regain his property.)
Crypto-Jews or marranos are usually defined as Jews who outwardly appeared to convert to Christianity (either because they were forced to under penalty of exile or death, or because it was the only way they could obtain education or employment), but who continued to practice Judaism in secret. Conversos is the term generally applied to people who converted to Christianity (freely or not) and who actually gave up the practice of Judaism. (These three terms are used interchangeably by some, but for my purposes here, I retain a distinction between secret Jews and former Jews.)
Crypto-Jews were fairly common in the middle ages, as persecution and forced conversion drove many Jews underground. Some re-merged with the wider Jewish community when they were able to escape or the persecution eased, but others slowly and inexorably slid into true conversion. Descendents of conversos are still found in Mexico and the Southwestern United States (most recently discovered in Hispanic Americans and Native Americans by the appearance of an aggressive breast cancer gene found predominately in Jewish women).
There has been a lot of speculation about what famous figures of the middle ages might have been secret Jews. Nostradamus had Jewish ancestors and was educated in Hebrew, Kabbalism, and other Jewish studies (he was almost certainly never a practicing Jew, though, and he had Christian and even Muslim leanings which probably made him a spiritualist more than anything).
Christopher Columbus has also been a source of speculation for years (and probably centuries). There is a very high probability that he was descended from Jews; the real question is whether or not he was a practicing Jew. Evidence has slowly been building, though, that he was probably a Crypto-Jew. Certainly his occupation befits a Crypto-Jew (although not all explorers were Jews, by any means). Several families of European Jews were, at the time, the best mapmakers in the world (Abraham Zacuto made maps for Columbus).
Machiavelli has also been a source of speculation, based on his writings. Like Nostradamus, though, he was probably not a practicing Jew (if he had any Jewish roots at all), but does seem to be learned in Jewish texts.
In fact, many dyed-in-the-wool Christians studied Jewish texts during the Renaissance, because Jews could publish tracts on theology and philosophy that were banned by the Church (I would offer up Leonardo da Vinci as an example of a Christian who studied Jewish texts, but there’s a question over whether or not his mother was Jewish). Jews also traveled and traded more extensively, and they were the first to import important books on medicine and astronomy from Muslim countries. Jews were also, on average, much more likely to read and write, which explains why they were often the repositories for information in Europe during the middle ages and the Renaissance.
For more articles and information on crypto-Jews of the middle ages and Renaissance, explore The Society for Crypto–Judaic Studies.