And do not marry polytheistic women until they believe. And a believing slave woman is better than a polytheist, even though she might please you. And do not marry polytheistic men [to your women] until they believe. And a believing slave is better than a polytheist, even though he might please you. Those invite [you] to the Fire, but Allah invites to Paradise and to forgiveness, by His permission. And He makes clear His verses to the people that perhaps they may remember. (Al Baqarah 2:221)
By the laws of Allah, Muslims are generally supposed to marry only Muslims. The injunctions against a Muslim marrying a non-Muslim are clearly set out in the Qur'an and there should be no further debate about it. There are certain exceptions for Muslim men as set out below:
And [lawful in marriage are] chaste women from among the believers and chaste women from among those who were given the Scripture before you, when you have given them their due compensation, desiring chastity, not unlawful sexual intercourse or taking [secret] lovers. (Al Maeda v 5:5)
When we see an Islamic regulation, we should ask the wisdom behind it, rather than question the validity of the regulation. The Almighty can question us, but not the other way around. Lately the topic of interfaith marriages has been up for debate, however every ruling in Islam has a wisdom, and our obedience to Allah should be above our loyalty to politics, personal beliefs and other ideologies.
In the case of women, the injunction against marrying a non-Muslim man is absolute. If she marries a non-Muslim man, scholarly consensus is that she is not only continuously committing the sin of fornication, but she will also fall out of Islam entirely for flouting Allah's laws. If she did this without knowledge or intention, she should leave her non-Muslim husband, make taubah and declare her shahadah afresh. This prohibition is unqualified and it does not matter if the husband is Christian, Hindu or atheist.
Muslim men, under some circumstances and subject to very strict stipulations, are given a license to marry chaste women who are descended from the People of the Book (practicing Christians and Jews who fulfil certain conditions). This exception comes with many conditions, but many men take advantage of the exception and ignore the conditions that accompany the marriage.
The predominant marriage structure universally is for the wife to take the lead from the husband. In most households, the husband is the head of the family. The husband is able to assert more authority over his wife than the other way around. Most women, whatever their religion and culture, are inclined to follow their husbands' wishes and goals in life.
The danger of a Muslim woman being in a non-Muslim marriage and potentially living with a non Muslim family unit (the in-laws) is that she would be placed in an environment where she is receiving no support for her journey in Islam. In fact, she could potentially face plenty of opposition from her husband and his family, and be placed under constant pressure to abandon her faith. There are also other issues, such as the upbringing of the children, in which she might not have a decisive say.
The children of such a marriage will adopt their surnames from the husband's side of the family, but whose deen will they inherit? What if the husband commands that the children follow his religion, what then of the responsibility of the Muslim mother towards the children's spiritual upbringing? Will she be in a dominant position to defy her husband and ensure that her children are brought up as Muslims, or would she be in a position of vulnerability or dependence (whether emotional, financial or physical) where she has to place her husband's wishes above hers? The results on her deen and her children's are potentially catastrophic, which is why Islam does not allow for Muslim women to be placed in such a position in the first place. It is not about equality and women's rights, but a far more important matter of protecting the Muslim woman's deen.
The early Muslim women left their non-believing husbands once it was decreed that a believing woman cannot be with a non-believing man. Their deen was more precious to them than their husbands, but that is not to say that they did not genuinely love their husbands or that the parting did not cause great heartbreak. Zainab AS, the daughter of the Prophet SAW, was one such lady. She emigrated from Mekah, leaving her non-believing husband behind. Although the love for each other continued to burn bright in their hearts, she knew they could not be together because obeying Allah in this matter took precedence over her love for him. After many painful years of separation, her husband eventually, of his own accord, took the shahadah, and only then were they reunited as husband and wife.
In the case of a Muslim husband, he is allowed to marry the People of the Book – the chaste, sincere and practising followers of the Abrahamic faith. Many aspects of their religion and way of life, if they were to apply the original teachings of Judaism and Christianity, are consistent with Islamic tenets. This special permission does not extend to Buddhists, Hindus, free thinkers or those practising religions apart from those observed by the People of the Book. However, in doing so, the marriage must with the intention of inviting such wife to Islam.