Islam was introduced into the region in about 630 when Prophet Muhammad was still alive. It was during this period that the mosques in al Janad and the great mosque in Sanaá were built. Virtually all citizens in Yemen today are Muslims, either of the Zaydi order of Shi'a Islam or the Shafi'i order of Sunni Islam, representing approximately 35 percent and 65 percent of the total population, respectively. There are also a few thousand Ismaili Muslims, mostly in the north.

The Zaydis of the northern highlands dominated politics and cultural life in northern Yemen for centuries; with unification, and the addition of the south’s almost totally Shafi'i population, the numerical balance has shifted dramatically away from the Zaydis. Nevertheless, Zaydis are still overrepresented in the government and, in particular, in the former North Yemeni units within the armed forces. However, there are no reported incidents of violence or discrimination between the adherents of the two main orders of Islam, Zaydi and Shafa'i, in Yemen. Except for a small politically motivated clerical minority, religiously motivated violence is neither incited nor tolerated by the Islamic clergy.

Status of Religious Freedom

The Constitution provides for freedom of religion, and the Government generally respects this right in practice; however, there were some restrictions. The Constitution declares that Islam is the state religion. Followers of other religions are free to worship according to their beliefs and to wear religiously distinctive ornaments or dress; however, the Government forbids conversions, requires permission for the construction of new places of worship, and prohibits non-Muslims from proselytizing and holding elected office. The Constitution states that Shari'a is the source of all legislation.

Under Islam as applied in the country, the conversion of a Muslim to another religion is considered apostasy, a crime punishable by death. There were no reports of cases in which the crime has been charged or prosecuted by government authorities. Public schools provide instruction in Islam but not in other religions. However, almost all non-Muslims are foreigners who attend private schools.

The Government monitors mosques for sermons that incite violence or other political statements that it considers harmful to public security. Private Islamic organizations may maintain ties to pan-Islamic organizations and in the past have operated private schools. However, in May 2001, the Government mandated the implementation of a 1992 law to unify educational curriculums and administration of all publicly funded schools. Publicly funded Islamic schools will be absorbed into the national system.

Shari'a-based law and social custom discriminate against women. Men are permitted to take as many as four wives, although very few do so. By law the minimum age of marriage is 15. However, the law largely is not enforced, and some girls marry as early as age 12.

Source : http://www.fact-archive.com/encyclopedia/Islam_in_Yemen