Short answer
It depends on what the person actually believes. Hanafi scholars of the Deobandi tradition say a nikah is not valid with someone who holds beliefs that take a person outside Islam, such as that the Quran was altered or that Aisha was guilty of what Allah cleared her of. Where none of those beliefs is held, the nikah is valid, but these scholars still strongly advise against it.
A nikah between a Sunni and someone who holds beliefs of kufr (disbelief), such as the Quran being altered, the divinity of Ali, or accusing Aisha of immorality, is not valid. A Shia who holds none of these is a Muslim and the nikah is valid, but because the differences in belief and family law are deep, most Deobandi muftis strongly discourage such a marriage.
The four schools agree on this.
Explanation
This is not a difference between the four Sunni schools. The question turns on belief, because a Muslim woman may not marry a man outside Islam, and a Muslim man may only marry a Muslim woman or a Christian or Jewish one. So the scholars do not judge a label. They ask what the person holds.
Ibn Abidin in Radd al-Muhtar names certain beliefs as kufr, such as ascribing divinity to Ali, claiming that Jibril erred in bringing the revelation, or accusing Aisha of the sin Allah declared her innocent of in Surah al-Nur. Later Hanafi muftis add the belief that the Quran has been altered. Allah says about that slander, "Allah warns you against returning to the likes of this, ever, if you should be believers" (24:17). Someone who holds such beliefs cannot marry a Sunni Muslim. Many Deobandi fatwas add that because some of these ideas are widely taught, a family should check carefully and not assume.
Some respected scholars, especially outside South Asia, view most ordinary Shia as Muslims and hold such marriages valid. Even they usually advise care, because husband and wife will differ in:
- How they pray, fast and which hadith they follow.
- Family law, including the permission of temporary marriage (mut'ah) in Shia law. See mut'ah.
- How the children will be raised, and how each family will feel about the other's practice.
The sunnah is to choose a spouse for religion, and that includes sharing it closely enough to build a home together. Speak to each other honestly about belief before any commitment, and ask a mufti who can hear the details. Calm discussion, not insults, is the way of the deen on this subject.
Ask a mufti about your own case. This answer explains the general ruling. When the details of your situation matter, as they do here, ask a qualified mufti and give them the full facts. Where to ask a scholar.
The evidence
Quran, Surah An-Noor 24:17
يَعِظُكُمُ ٱللَّهُ أَن تَعُودُوا۟ لِمِثْلِهِۦٓ أَبَدًا إِن كُنتُم مُّؤْمِنِينَ ﴿١٧﴾
Allah warns you against returning to the likes of this [conduct], ever, if you should be believers.
Allah declared Aisha innocent and warned against repeating the slander.
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.
Faith is a condition in choosing a spouse.
Sahih al-Bukhari 5090
Sahih... فاظفر بذات الدين تربت يداك ...
"So win the one with religion, may your hands be covered in dust."
Religion is the first thing to look for in a spouse.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Nikah, Fasl fi al-Muharramat. Discusses those whose beliefs amount to kufr, such as accusing Aisha or claiming divinity for Ali, and the effect on marriage.
- Imdad al-Fatawa (Ashraf Ali Thanawi), Kitab al-Nikah, on marriage with Shia. Rules according to the person's beliefs.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. 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.