Salafi Islam is the state religion of Saudi Arabia. The connection between Islam and modern-day Saudi Arabia is uniquely strong. The kingdom is called the "home of Islam"; the kingdom is also the birthplace of the Islamic Prophet Muhammad, with all territories of Saudi Arabia and the Arabian Peninsula being united and ruled by him. It is the location of the cities of Mecca and Medina, where Muhammad, the messenger of the Islamic faith, lived and died. The kingdom attracts millions of Muslim Hajj pilgrims annually, and thousands of clerics and students who come from across the Muslim world to study. The official title of the King of Saudi Arabia is "Custodian of the Two Holy Mosques"—the two being Al-Masjid al-Haram in Mecca and Al-Masjid al-Nabawi in Medina—which are considered the holiest in Islam. In the 18th century, a pact between Islamic preacher Muhammad ibn Abd al-Wahhab and a regional emir, Muhammad bin Saud, brought a new form of Islam (Salifism) of Sunni Islam first to the Najd region and then to the Arabian Peninsula. Referred to by supporters as "Salafism" and by others as "Wahhabism", this interpretation of Islam became the state religion and interpretation of Islam espoused by Muhammad bin Saud and his successors (the Al Saud family), who eventually created the modern kingdom of Saudi Arabia in 1932. The Saudi government has spent tens of billions of dollars of its petroleum export revenue throughout the Islamic world and elsewhere on building mosques, publishing books, giving scholarships and fellowships, hosting international Islamic organisations, and promoting its form of Islam, sometimes referred to as "petro-Islam". The Wahhabi mission has been dominant in Najd for two hundred years, but in most other parts of the country—Hejaz, the Eastern Province, Najran—it has dominated only since 1913–1925. Most of the 15 to 20 million Saudi citizens are Sunni Muslims, while the eastern regions are populated mostly by Twelver Shia, and there are Zaydi Shia in the southern regions. According to a number of sources, only a minority of Saudis consider themselves Wahhabis, although according to other sources, the Wahhabi affiliation is up to 40%, making it a very dominant minority, at the very least using a native population of 17 million based on "2008–09 estimates". In addition, the next largest affiliation is with Salafism, which encompasses all of the central principles of Wahhabism, with a number of minor additional accepted principles differentiating