The popular viewpoint amongst Shia scholars and jurisprudents is that a Shia man can get married to a Sunni woman, but there is a difference of opinion regarding the marriage of a Sunni man with a Shia woman; most scholars see it as impermissible.
The reasons behind the first verdict:
Being equals which is one of the conditions of marriage means for the two to be married to both be Muslim.
What can be deduced by the many hadiths on the subject of Sunnis and Shias getting married is that being Muslim only depends on one saying the shahadatain; hence the permissibility of getting married with them and them being tahir.
Of course, Shia jurisprudents have issued obligatory precautions on the marriage of a Sunni man with a Shia woman, because women are usually influenced by their spouses and there are great chances of her denouncing her true beliefs and faith after a while into the marriage.
There are different viewpoints on this issue in the Shia school of thought. The popular one amongst scholars is that it is permissible. However, if a Sunni man wants to get married to a Shia girl, the popular verdict is that is impermissible. Here we will address the question on a Sunni girl getting married to a Shia man.
The Reasons Behind This Fatwa
1- Being equals in marriage
Being equals when getting married is an issue that has been addressed in both Shia and Sunni fiqh books and has its own chapter in them [which shows its significance]. The reason for its significance is the different important outcomes that it entails. What is meant by being equals is that the two to be married must be equal in Islam, meaning that they both have to at least be Muslim. Therefore, for the girl to be a Sunni and the man to be a Shia is enough since they are both Muslim.
There are various hadiths on these types of marriages (where the two aren’t the same in being Shia or Sunni). What can be understood by these hadiths is that being Muslim only depends on saying the shahadatain [although being Mu’min (Iman) needs more than that, namely to believe in the twelve imams’ succession to the prophet (pbuh)]; one who is Muslim can be married with and is tahir. Here are two examples of these hadiths:
a) In a hadith, a person asks Imam Sadiq (as) about the difference between Islam and Iman [Belief/Faith]. The imam responds with the following: “Islam is what the people practice on the outside; by testifying that there is only one God that can be worshipped and that Muhammad (pbuh) is his servant and messenger, praying, giving charity, pilgrimage to Makkah and fasting during the holy month of Ramadhan. This is Islam. Iman encompasses all of these outer rituals plus believing in this matter (what the imam means by “this matter” is the important matter of wilayah or the authority of the imams and their succession to the prophet (pbuh)). So if a person possesses the first, yet lacks the latter, he/she is both Muslim and astray.”
b) Hamran ibn A’yan narrates from Imam Baqir (as) that he said: “Iman is the thing that settles in the heart and drives the individual towards Allah (swt) and His obedience…While Islam has to do with the outside of individuals; it is something that all sects and people possess and is the subject of many Islamic laws such as preserving lives, inheritance and marriage…”
These hadiths clearly state that Sunnis are Muslims and this ruling applies to all Islamic sects, hence the permissibility of marriage between Sunnis and Shias.
When the imam says that the permissibility of marriage is one of the rulings that pertains to this issue, it is absolute, therefore it doesn’t make a difference if the man is a Shia and the girl is Sunni or vice versa.
Of course, Shia scholars have prohibited the marriage in the case of the girl being Shia and the man being Sunni, because of a secondary reason which is that women are usually influenced by their husbands and there are great chances of the wife denouncing her true beliefs after a while. As a result, scholars have issued an obligatory precaution on the issue [meaning that it isn't permissible unless one finds another marja’ that allows it].
Question: What is the ruling on the marriage between a Muslim and non-Muslim, and the ruling on marriage between a Shia and Sunni?
The late Grand Ayatullah Fazel Lankarani: Marriage between Muslims and non-Muslims is void, the marriage of a Shia woman and Sunni man is makruh, the temporary marriage of a Muslim man and non-Muslim woman is permissible and the marriage of a Shia man and Sunni woman is permissible.
The Grand Ayatullah Bahjat: The first case (Muslim with non-Muslim) is only permissible if the non-Muslim is of The People of the Book and the marriage is temporary [mutah], and the second case (Shia woman and Sunni Man) isn't permissible as a taklifi precaution.
The Grand Ayatullah Sistani: It is an obligatory precaution not to contract marriage with The People of the Book, and the marriage of Shias and Sunnis is permissible, given that one doesn’t fear for their beliefs.
The Grand Ayatullah Makarm Shirazi: The marriage of Muslims with non-Muslims is impermissible and the marriage of Shia men with Sunni women is permissible, but the marriage of Shia women with Sunni men is impermissible, considering the fact that there are great chances of her losing her faith.
 Naser Makarem Shirazi in his fiqh kharej lessons at the seminary of
 Naser Makarem Shirazi in his kharej fiqh lessons.
 Muhammad ibn Yaqub Kuleini, Usul Kafi (translation and commentary of Seyyid Jawad Mustafavi), vol.3, pg.39.
 Usul Kafi, vol.3, pg.42, hadith 42.
 Naser Makarem Shirazi in his kharej fiqh lessons.
 With help from: Index 1253 (website: ), detailed answer.