The Rama (Rabbi Moshe Isserles of Cracow, 1525-1572), in his glosses to the Eben Ha’ezer section of the Shulhan Aruch (Siman 4), rules that it is forbidden to marry somebody from the heretical Karaite sect. The Karaites were a group of Jews that denied the authority of the oral tradition recorded in the Talmud, including the Sages’ interpretations of the Torah and the Halachic system derived from these interpretations. They insisted on following only the plain reading of the Torah, without paying any attention to the Rabbinic Halachic tradition. This rejection of the authority of the Talmud resulted in improper procedures for divorce. When a Karaite couple divorced, they did not follow the rules and guidelines established by Halacha, and they therefore were not, technically speaking, divorced. If the wife then remarried, her relationship with her second husband was considered adulterous, since she was still married to the first husband according to Halacha. Hence, the children born from the second marriage had the status of Mamzerim, who are forbidden for marriage. And if a Mamzer marries, the children also have the status of Mamzerim.
As a result, the Karaite community had many Mamzerim. The Rama therefore ruled that it is forbidden to marry somebody who descends from this community, even if that individual rejected the heretical beliefs of the Karaites and embraced traditional Judaism, given the likelihood that this person is a Mamzer.
A different view on this subject was taken by the Radbaz (Rabbi David Ben Zimra of Egypt, 1480-1574), in his responsa (vol. 1, Siman 73), where he writes that one may marry somebody from the Karaite sect, assuming, of course, that this person has embraced traditional Judaism. The Radbaz claimed that although the Karaites’ divorces are invalid, their marriages are also invalid. The witnesses at Karaite weddings were, more often than not, Karaites, who are ineligible to serve as witnesses since they do not believe in or follow Halacha. Hence, since divorced Karaite couples were never Halachically married to begin with, a divorced Karaite woman’s second marriage is not considered an adulterous relationship, and does not produce Mamzerim.
The Bet Yosef (commentary by Maran, author of the Shulhan Aruch), however, cites Rabbenu Shimshon as ruling stringently on this issue, and, as mentioned the Rama rules against the Radbaz and forbids marrying a member or descendant of the Karaite sect.
The Bet Yosef and Rama also discuss the question of marrying somebody from the Marranos, the Jews in Spain who outwardly conducted themselves as gentiles to avoid religious persecution. These authorities rule that it is permissible to marry somebody from this group, since they strictly observed the laws of divorce. Although the Marranos acted as gentiles in public, they remained faithful to Jewish practice in their private lives, including with regard to marriage and divorce. Therefore, we need not be concerned about the possibility of a Marrano being a Mamzer, and they are thus permissible for marriage, assuming, of course, that they have repented and conduct themselves as Jews both in private and public.
This discussion underscores the delicate nature of Gittin (the Halachot of divorce) and the importance of ensuring that divorces are properly handled by competent Rabbinic scholars. The laws of Gittin are especially intricate, and even an incorrect letter on the bill of divorce can invalidate the divorce, which will have the effect of rendering the woman’s second marriage an adulterous relationship and her children Mamzerim. When, Heaven forbid, a divorce becomes necessary, the couple must consult with a qualified Rabbi to ensure that it is done according to Halacha, and they should not simply "shop around" for the cheapest Rabbi they can find to handle the complex process of divorce. And if a person knows a couple that is in the process of divorcing, he should encourage them to have it handled properly according to Halacha, in order to avoid Mamzerut.
