Medieval Iberia is an absolutely fascinating case study of interfaith tensions and diplomacy which was often informed by sex, gender, and interfaith marriages.

The first thing you should know is the myth behind the Umayyad conquest of the Iberian peninsula in 711. According to legend, there was a Christian count in North Africa called Julian. Julian sent his daughter to the court of King Roderic of Visigothic Spain to be educated. When Roderic raped Julian's daughter, Count Julian betrayed him and invited the Muslims to invade the peninsula.

Already, we see the influence of sex, interfaith, and intrafaith tension come into play. The rape of a woman is analogous to the penetration/rape of the peninsula. Some stories portray the daughter as a vixen who deliberately seduced Roderic, while others assert that Roderic was wicked and depraved. In any case, it's interesting that the conflict arose as an intrafaith conflict between two Christians, rather than between Christians and Muslims. This will become significant later.

During the coexistence of Jews, Muslims, and Christians in medieval Spain, there were varying degrees of tension and harmony. In most places under Islamic rule, Jews and Christians were nominally protected as "people of the book." They paid extra taxes but were allowed to practice their own religion and build churches and synagogues. Arabic and Romance dialects were the most common languages used to communicate amongst all faiths. There were even Arabized Christians, called Mozarabs, who spoke Arabic and dressed in the Islamic fashion.

So on an individual level, many people were able to live in relative peace. Keep in mind that pogroms weren't uncommon and there were sometimes public burnings of the sacred texts of the different religions. This was during the time of the Reconquista after all. Christian rulers in the northern mountains continually tried to push their borders further south. They were sometimes successful, but the Reconquista wasn't the pure ideological religious warfare we read about in textbooks. It wasn't purely a case of Christians vs. Muslims. Often, a Christian ruler would ally himself with a Muslim ruler to defeat a fellow Christians rival. It was a time of opportunism and constantly shifting alliances between Muslims and Christians.

In the midst of the Reconquista, members of other faiths often proved valuable intellectual resources and courtiers, particularly Jews. Jews were regarded as skilled scholars, translators, doctors, and astrologers by both Muslim and Christian rulers. Islamic fashion and architecture also became prized and imitated in Christian courts even as they fought vicious holy wars allegedly to bring Andalusia back under Christian control. It became a form of intellectual and cultural warfare to appropriate the language, architecture, cultural style, and knowledge of the comparing culture as a display of cultural power and dominance. The goals of such cultural and intellectual exchanges were often explicitly conversionary. For instance, the Abbot of Cluny commissioned a Latin translation of the Qu'ran for the express goal of converting Muslims to Christianity. There were also public disputations in which Christians and Jews made public defenses of their faiths in an attempt to convert Jews to Christianity through debate. Perhaps the most famous of these debates is the Disputation of Barcelona in 1263 in which Christians were declared the winner despite convincing arguments by Nachmanides.

There was massive anxiety about conversion in medieval Spain. Conversions between all three faiths were quite common, sometimes forced, and often aligned with whichever powers currently controlled one's city or the fate of one's family. Some of the most common conversion accounts concern servants converting to the faith of their masters. In contrast, there were also accounts of crypto practitioners of all three religions. Women in particular were known for continuing to practice their faith behind closed doors, including butchering their own meat and performing ritual bathing. Women were also known to hide religious texts in the folds of their skirts during interrogations, both to protect their families abd preserve their connection to their faith.

There were also several legal measures specifically designed to make a person's faith obvious upon inspection, because you couldn't always tell from language or dress alone. I've come across texts with specific instructions for circumcision and inspecting men's genitalia to determine their faith. In other cases, Jews and Muslims had their hair and beards unceremoniously shaved.

Despite these tensions, interfaith marriage was more common than you might expect. Interfaith marriages were sometimes brokered as part of diplomatic negotiations, and conversion was sometimes a prerequisite for these matches. For the Umayyads, interfaith marriage was a savvy political tool that helped them consolidate dynastic power and minimize internal threats. As Simon Barton has convincingly argued, claims from other Muslim families were a massive threat in the Umayyads' tenuous empire. Christian marriages solidified alliances with Christian powers and also neutralized the threat of other Muslim families gaining too much power through royal matches. The practice of taking concubines and slaves from other faiths was even more common.

As time wore on, the lines between Christian, Jewish, and Muslim became blurrier and blurrier. Most people could speak the same language and dressed similarly. Skin tone and physical appearance had never been a good indicator and was even less so after several decades of intermarrying and conversion. With crypto religious practices and conversion so common, it grew increasingly likely that any given person might have ancestors from more than one faith.

This anxiety about religious identity and mixing was very evident in early modern Spanish literature, in which Moorish and Jewish prostitutes were perceived as spreading syphilis and moral depravity to their clients of all faiths and backgrounds.

I've typed this all on mobile so apologies for any spelling or formatting errors!

If you're interested in interfaith marriage in medieval Iberia I highly recommend the works of Simon Barton, particularly Conquerors, Brides, and Concubines.