Interfaith marriage, sometimes called a "mixed marriage", is marriage between spouses professing different religions. Although interfaith marriages are most often contracted as civil marriages, in some instances they may be contracted as a religious marriage. This depends on religious doctrine of the two party's religions; some of which prohibit interfaith marriage, but others allow it in limited circumstances. Several major religions are mute on the issue, and still others allow it with requirements for ceremony and custom. For ethno-religious groups, resistance to interfaith marriage may be a form of self-segregation. In an interfaith marriage, each partner typically adheres to their own religion, but an important point is in what faith the children will be raised.
According to Article 16 of the Universal Declaration of Human Rights, men and women who have attained the age of majority have the right to marry "without any limitation due to race, nationality or religion". Although most of Article 16 is incorporated verbatim in Article 23 of the International Covenant on Civil and Political Rights, the references to religious and racial limitations is omitted. Article 17, clause two of the American Convention on Human Rights says that all men and women have the right to marry, subject to the conditions of domestic law "insofar as such conditions do not affect the principle of nondiscrimination established in this Convention."
Interfaith marriage in Judaism was historically viewed with disfavor by Jewish leaders, and it remains controversial. The Talmud and poskim prohibit non-Jews to marry Jews, and discuss when the prohibition is from the Torah and when it is rabbinical. In 1236, Moses of Coucy encouraged Jewish men who had married Christian or Muslim women to divorce them. In 1844, the reform Rabbinical Conference of Brunswick permitted Jews to marry "any adherent of a monotheistic religion" if children of the marriage were raised Jewish. This conference was controversial; one of its resolutions called on members to abolish the Kol Nidre prayer, which opens the Yom Kippur service. One member of the conference later changed his opinion, becoming an opponent of intermarriage.
Traditional Judaism does not consider marriage between a Jew by birth and a convert as intermarriage; Biblical passages which apparently support intermarriage, such as that of Joseph to Asenath and Ruth to Boaz, were regarded by classical rabbis as having occurred after the non-Jewish spouse had converted. Some still considered Canaanites forbidden to marry even after conversion, although this did not necessarily apply to their children.
Orthodox Judaism refuses to accept intermarriage, and tries to avoid facilitating them. Conservative Judaism does not sanction intermarriage, but encourages acceptance of the non-Jewish spouse by the family in the hope that such acceptance will lead to the spouse's conversion to Judaism. In December 2014 the United Synagogue of Conservative Judaism's United Synagogue Youth controversially modified a binding rule that its leaders would not date non-Jews, replacing it with a "recogni[tion of] the importance of dating within the Jewish community."
Reform and Reconstructionist Judaism do not generally regard the authority of classical rabbis; many rabbis from these denominations are willing to officiate at interfaith marriages, although they try to persuade intermarried couples to raise their children as Jews. In 1870, some Reform Jews published the opinion that intermarriage is prohibited.
In 2015 the Reconstructionist Rabbinical College voted to accept rabbinical students in interfaith relationships, making Reconstructionist Judaism the first movement within Judaism to allow rabbis to have relationships with non-Jewish partners. Humanistic Judaism is a nontheistic alternative in contemporary Jewish life, defining Judaism as the cultural and historical experience of the Jewish people. The Society for Humanistic Judaism answers the question, "Is intermarriage contributing to the demise of Judaism?" on its website: "Intermarriage is the positive consequence of a free and open society. If the Jewish community is open, welcoming, embracing, and pluralistic, we will encourage more people to identify with the Jewish people rather than fewer. Intermarriage could contribute to the continuity of the Jewish people."
During the early 19th century, intermarriage was relatively rare; less than one-tenth of one percent of the Jews of Algeria, for example, practiced exogamy. Since the early 20th century, rates of Jewish intermarriage have increased. In the United States from 1996 to 2001, nearly half (47 percent) of marriages involving Jews were intermarriages with non-Jewish partners (a similar proportion—44 percent—as in the early 20th century in New South Wales).
In Hinduism, spiritual texts like Vedas and Gita do not speak of caste and interfaith marriages. However, law books like Manusmriti, Yajnavalkya Smriti, and Parashara speak of marriage rules among various kulas and gotras. Manusmriti versions are numerous as the original is not preserved and also represents one of the oldest attempts to formally regulate the secular society of India. It is not a religious text. According to the varna system, marriage is normally between two individuals of the same varna. Ancient Hindu literature identified four varnas: Brahmins, Kshatriyas, Vaishyas and Shudras. In ancient India, this varna system was strictly professional division based on one's profession. With time, it became a birthright. According to Manusmriti, partners in an intra-gotra marriage should be shunned as it is an equivalent of a sibling marriage shunned. In rural parts of modern-day India, which is mainly conservative, follows this rule, while Hindus living in the cities and abroad often accept the inter-caste marriage.
Some gurdwaras allow weddings between a Sikh and a non-Sikh, but others oppose it. In 2014, the Sikh Council in the UK developed a consistent approach towards marriages in Gurdwaras where one partner is not of Sikh origin, following a two-year consultation with Gurdwara Sahib Committees, Sikh Organisations, and individuals. The resulting guidelines were approved by the General Assembly of Sikh Council UK on 11 October 2014, and state that Gurdwaras are encouraged to ensure that both parties to an Anand Karaj wedding are Sikhs, but that where a couple chooses to undertake a civil marriage they should be offered the opportunity to hold an Ardas, Sukhmani Sahib Path, Akhand Path, or other service to celebrate their marriage in the presence of family and friends. Some gurdwaras permit mixed marriages, which has led to controversy.
Some traditional Zoroastrians in India disapprove of and discourage interfaith marriages, and female adherents who marry outside the faith are often considered to be excommunicated. When a female adherent marries a partner from another religion, they go through the risk of not being able to enter the Agyaris and Atash Behrams. In the past, their partner and children were forbidden from entering Zoroastrian religious buildings; this is often still observed. A loophole was found to avoid such expulsion: the offspring (especially born out of wedlock) of a Parsi man and a non-Parsi woman were often "adopted" by the Parsi father and tacitly accepted into the religion. Alternatively in a few cases such as that of Suzanne RD Tata, the non-Zoroastrian spouse has been allowed to convert Zoroastrianism by undergoing the navjote ritual  Interfaith marriages may skew Zoroastrian demographics, since the number of adherents is low.
According to Indian law (where most Parsis live), only the father of the child must be a Zoroastrian for the child (or children) to be accepted into the faith. This has been debated, since the religion promotes gender equality (which the law violates). Zoroastrians in North America and Europe defy the rule, and children of a non-Zoroastrian father are accepted as Zoroastrians.
Some Christian denominations forbid interfaith marriage, citing 2 Corinthians 6:14 (although 1 Corinthians 7:14 allows it depending on the interpretations) and Deuteronomy 7:3 (depending on interpretation). In the Catholic Church, canon law deals with mixed marriages (a marriage between a Catholic and a baptized person outside the Church) and marriages in disparity of cult (marriage between a Catholic and an unbaptized person). Distinction is made between inter-denominational and interfaith marriage, and some denominations extend their own rules and practices to other Christian denominations.
A primary Islamic legal concern is that the offspring of an interfaith marriage between a Muslim a non-Muslim are to be Muslim offspring, and raised as such. Sharia, thus, has differing regulations on interfaith marriage, depending on, firstly, what is the gender of the prospective intermarrying Muslim, and secondly, what non-Muslim religion is adhered to by the person that a Muslim is seeking to intermarry with.
While Islamic Law permits a Muslim man to marry up to four women, the preference is that one or all of his wives be Muslim. If he intermarried with a non-Muslim, one or more of the four allowed wives may be non-Muslim women provided that they are from among the People of the Book (i.e. female Christians or female Jews). Additionally, they must have been chaste, and all children must be brought up Muslim. Beyond this exemption, a Muslim man may not intermarry with females who are not from among the People of the Book unless they convert to Islam (which is not required of Christian females and Jewish females). Thus, Muslim men are prohibited from intermarrying, for instance, Hindus, Sikhs, Buddhists, etc., as well as pagans or atheists, unless the woman converts to Islam. If they did, however, convert, it would no longer be considered intermarriage, but a marriage between Muslims, and thus not prohibited.
According to traditional understandings of interfaith marriage in Islam, Muslim women are forbidden from intermarrying based on Islamic law. This is understood to be irrespective of whether or not she wishes to marry a male from among the People or the Book (i.e. a male Christian or Jew) or a male of any other religion. Based on this interpretation, this would not apply if the non-Muslim man converted to Islam, as the Muslim woman would no longer be considered to be intermarrying, but marrying a Muslim man. Additionally, she may only be married to one Muslim man at any one time (i.e. she may not have multiple husbands at the same time). However, there is no clear prohibition of Muslim women marrying non-Muslim People of the Book in the Qur'an because the Qur'an does not address the issue as explicitly as it does with Muslim men being permitted to marry People of the Book. Moreover, there are multiple cases throughout the Qur'an in which the text only refers to men but it is implied that the verse also applies to women. In fact, some religious scholars, such as Kecia Ali, would claim that to interpret the Qur'an as prohibiting women from interfaith marriage is itself a "significant interpretative leap, going beyond the verse itself." 
Early jurists in the most-prominent schools of Islamic jurisprudence ruled in fiqh that the marriage of a Muslim man to a Christian or Jewish woman is makruh (disapproved) if they live in a non-Muslim country. Umar (634–644) denied interfaith marriage to Muslim men during his command of the ummah. According to the Quran,
Today the good things are made lawful for you, and the food of the ones to whom the Book was brought is lawful to you, and your food is made lawful to them. And (so) are believing women in wedlock, and in wedlock women of (the ones) to whom the Book was brought even before you when you have brought them their rewards in wedlock, other than in fornication, neither taking them to yourselves as mates (i.e., girl-friends). And whoever disbelieves in belief, (i.e., the religion) then his deed has been frustrated and in the Hereafter, he is among the losers. (Surah 5:5)
Scholar Ahmad Kutty of Toronto has expressed disapproval of interfaith marriage, citing Umar. According to scholar Bilal Philips, the verse permitting Muslim men to marry non-Muslim women is no longer valid for several reasons (including its misinterpretation). Canadian Islamic scholar Shabir Ally has also said that it is makruh for a Muslim man to marry outside his religion. This prohibition preserves and expands Islam in patriarchal, multi-faith societies. It ensures that over a number of generations, Islam would gain in numbers relative to other religions.
If a non-Muslim woman married to non-Muslim converts to Islam, the marriage is suspended until her husband converts to Islam; she could theoretically leave the non-Muslim husband and marry a Muslim one, analogous to the Pauline privilege for Catholics. If the non-Muslim husband converts, a new marriage is not needed. According to the Quran,
O ye who believe! When there come to you believing women refugees, examine (and test) them: Allah knows best as to their Faith: if ye ascertain that they are Believers, then send them not back to the Unbelievers. They are not lawful (wives) for the Unbelievers, nor are the (Unbelievers) lawful (husbands) for them. But pay the Unbelievers what they have spent (on their dower), and there will be no blame on you if ye marry them on payment of their dower to them. But hold not to the guardianship of unbelieving women: ask for what ye have spent on their dowers, and let the (Unbelievers) ask for what they have spent (on the dowers of women who come over to you). Such is the command of Allah. He judges (with justice) between you. And Allah is Full of Knowledge and Wisdom. (Surah 60:10)
According to the Bahá'í Faith, all religions are inspired by God and interfaith marriage is permitted. A Bahá'í ceremony should be performed with the non-Bahá'í rite (or ceremony). If both ceremonies are performed, the non-Bahá'í ceremony should not invalidate the Bahá'í ceremony; the Bahá'í partner remains a Bahá'í, and is not adopting the religion of the other partner in the ceremony. The Bahá'í partner should also abstain from vows (or statements) committing them to a declaration of faith in another religion or that are contrary to the principles of the Bahá'í Faith. The two ceremonies should be performed on the same day; their order is not important. The Bahá'í ceremony may be performed in the place of worship of the other religion if it is afforded respect equal to the non-Bahá'í ceremony and is clearly distinct from the non-Bahá'í ceremony.
In orthodox Serer religion (an ethnoreligious faith), interfaith and interracial marriages are forbidden. Banishment and disinheritance may be levied against a Serer who disobeys the law. The Serer-Noon (a sub-group of the Serer people) adhere strongly to this teaching.
In modern times various composers have written sacred music for use during interfaith marriage ceremonies including: