Short answer
Because Allah has forbidden it: a Muslim woman's nikah with any non-Muslim man, including a Christian or Jew, is void, and the four schools agree. The man must sincerely accept Islam first. The wisdom scholars mention is the protection of her faith and her children's faith, since the husband leads the home and children usually follow the father.
It is haram and the nikah is void (batil). Relations under such a "marriage" are not lawful. If a non-Muslim man sincerely becomes Muslim, a new nikah can then be done.
The four schools agree on this.
Explanation
Allah says of believing women and disbelieving men: "they are not lawful wives for them, nor are they lawful husbands for them" (60:10). In 2:221 He forbids marrying Muslim women to men who associate partners with Him until they believe. The scholars of all four schools, and the whole Muslim community through history, agree that a Muslim woman may only marry a Muslim man.
Why is the rule different for men? Scholars give several reasons, while noting that the first reason is simply that Allah commanded it:
- Respect for her faith. A Muslim husband believes in Musa and Isa, peace be upon them, and in the Torah and the Gospel as revealed books, so he is bound to respect a Christian or Jewish wife's prophets. A non-Muslim husband does not believe in the Prophet Muhammad ﷺ or the Quran, and she has no such protection.
- Leadership of the home. The husband carries responsibility and authority in the family (4:34). A believing woman is not placed under the authority of one who rejects her faith.
- The children. Children generally carry their father's name and are raised in his religion.
If you love someone who is not Muslim, the only halal path is that he learns about Islam and accepts it sincerely, for its own sake. A "conversion" only on paper does not change the ruling before Allah. If a married non-Muslim woman becomes Muslim and her husband does not, the marriage is affected and she should ask a mufti about her case. See also marrying a Christian or Jewish woman and the new Muslim guide.
The evidence
Quran, Surah Al-Mumtahana 60:10
يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوٓا۟ إِذَا جَآءَكُمُ ٱلْمُؤْمِنَٰتُ مُهَٰجِرَٰتٍۢ فَٱمْتَحِنُوهُنَّ ۖ ٱللَّهُ أَعْلَمُ بِإِيمَٰنِهِنَّ ۖ فَإِنْ عَلِمْتُمُوهُنَّ مُؤْمِنَٰتٍۢ فَلَا تَرْجِعُوهُنَّ إِلَى ٱلْكُفَّارِ ۖ لَا هُنَّ حِلٌّۭ لَّهُمْ وَلَا هُمْ يَحِلُّونَ لَهُنَّ ۖ وَءَاتُوهُم مَّآ أَنفَقُوا۟ ۚ وَلَا جُنَاحَ عَلَيْكُمْ أَن تَنكِحُوهُنَّ إِذَآ ءَاتَيْتُمُوهُنَّ أُجُورَهُنَّ ۚ وَلَا تُمْسِكُوا۟ بِعِصَمِ ٱلْكَوَافِرِ وَسْـَٔلُوا۟ مَآ أَنفَقْتُمْ وَلْيَسْـَٔلُوا۟ مَآ أَنفَقُوا۟ ۚ ذَٰلِكُمْ حُكْمُ ٱللَّهِ ۖ يَحْكُمُ بَيْنَكُمْ ۚ وَٱللَّهُ عَلِيمٌ حَكِيمٌۭ ﴿١٠﴾
O you who have believed, when the believing women come to you as emigrants, examine them. Allah is most knowing as to their faith. And if you know them to be believers, then do not return them to the disbelievers; they are not lawful [wives] for them, nor are they lawful [husbands] for them. But give the disbelievers what they have spent. And there is no blame upon you if you marry them when you have given them their due compensation. And hold not to marriage bonds with disbelieving women, but ask for what you have spent and let them ask for what they have spent. That is the judgement of Allah; He judges between you. And Allah is Knowing and Wise.
Believing women are not lawful for disbelieving men.
Quran, Surah Al-Baqara 2:221
وَلَا تَنكِحُوا۟ ٱلْمُشْرِكَٰتِ حَتَّىٰ يُؤْمِنَّ ۚ وَلَأَمَةٌۭ مُّؤْمِنَةٌ خَيْرٌۭ مِّن مُّشْرِكَةٍۢ وَلَوْ أَعْجَبَتْكُمْ ۗ وَلَا تُنكِحُوا۟ ٱلْمُشْرِكِينَ حَتَّىٰ يُؤْمِنُوا۟ ۚ وَلَعَبْدٌۭ مُّؤْمِنٌ خَيْرٌۭ مِّن مُّشْرِكٍۢ وَلَوْ أَعْجَبَكُمْ ۗ أُو۟لَٰٓئِكَ يَدْعُونَ إِلَى ٱلنَّارِ ۖ وَٱللَّهُ يَدْعُوٓا۟ إِلَى ٱلْجَنَّةِ وَٱلْمَغْفِرَةِ بِإِذْنِهِۦ ۖ وَيُبَيِّنُ ءَايَٰتِهِۦ لِلنَّاسِ لَعَلَّهُمْ يَتَذَكَّرُونَ ﴿٢٢١﴾
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.
Muslim women are not given in marriage to non-Muslim men.
Where the ruling is recorded
- al-Hidaya, Kitab al-Nikah, Fasl fi Bayan al-Muharramat. A Muslim woman's marriage to a non-Muslim is not valid.
- Bada'i al-Sana'i, Kitab al-Nikah, on the condition of Islam in the husband. Explains the reasons for the rule.
- al-Mughni (Ibn Qudama), Kitab al-Nikah. Records the consensus that a Muslim woman may not marry a non-Muslim.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.