One of the conditions for a nikah to be valid is to have witnesses. If the witnesses were Non-Muslims, will the nikah be valid?
According to the Hanafi school of thought, the witnesses of a Muslim marriage must be Muslim and otherwise the Nikah is not valid.
The exception is the nikah of a Muslim man with a Jewess or Christian subject of an Islamic state (Dhimmi), where there is a difference of opinion: The view recorded from Abu Hanifa and Abu Yusuf is that the Nikah will be valid even if the witnesses are Jewish\Christiah, however the opinion of Muhammad and Zufar is that it is not valid.
Quote From Hidayah:
The contract of nikah of Muslims is not concluded unless there are present two Muslim, free, major and sane male witnesses, or one male and two women.
If a Muslim marries a Dhimmi woman with the marriage witnessed by two Dhimmis, it is valid according to Abu Hanifah and Abu Yusuf (God bless them). Muhammad and Zufar (God bless them) said that it is not permitted.
The same is noted in Mukhtasar Al-Qudri.
The Shafis also require the witnesses to be Muslims, and my understanding is that they do not make an exception for Kitabee marriages.
الفصل الثالث : أنه لا ينعقد إلا بشهادة مسلمين ، سواء كان الزوجان مسلمين ، أو الزوج وحده . نص عليه أحمد . وهو قول الشافعي . وقال أبو حنيفة : إذا كانت المرأة ذمية ، صح بشهادة ذميين
( 5140 )
Marriage cannot be done except with two Muslim witnesses, whether the couple are both Muslims, or only the husband is Muslim. This was stated by Ahmad, and it is the view of al-Shaafa’i [ my own translation after this ] Abu Hanifa said that if the woman was a Dhimmi it is valid for the witnesses to be Dhimmi