The following is the stance of Ayatollah Syed Sistani with regards to marrying someone who is not a Muslim:
2406. A Muslim woman cannot marry a non-Muslim, and a male Muslim also cannot marry a non-Muslim woman *who are not Ahlul Kitab*. However, there is no harm in contracting temporary marriage with Jewish and Christians women, but the obligatory precaution is that *a Muslim should not take them in permanent marriage*. There are certain sects like Khawarij, Ghulat and Nawasib who claim to be Muslims, but are classified as non-Muslims.Muslim men and women cannot contract permanent or temporary marriage with them.