Jafari Shia Islam has been the official religion of Iran since the 16th century. Followers of Shia Islam differ with Sunni Muslims in beliefs (see Sunni Islam, Sunni form the majority of Muslims in the Middle East and the Islamic world) over the rightful succession to the Prophet Muhammad (PBUH), the founder of Islam. Iran’s 1979 constitution assigns to the Shia clergy important political leadership roles in the government. An estimated 93 percent of all Iranians follow Shia Islam, and nearly all are members of the Jafari group. Because Jafaris’ believe there are 12 legitimate successors, or imams of Muhammad (PBUH), they are often called Twelvers. Most of the remaining religious population belongs to other Islamic denominations, primarily Sunni Islam. In towns where there is mixed Muslim communities, some religious tensions have surfaced, especially during major religious observances. Sufism, or Islamic mysticism, is popular among Shia and Sunni Muslims seeking spiritual interpretations of religion. Iran also has small communities of Armenian and Assyrian Christians, Jews, and Zoroastrians. The Baha’i faith, which originated in Iran during the 19th century, has several thousand followers, since it is considered controversial for many years by members within Iranian parliament.