Islam is the largest religion in Nigeria and the country has the largest Muslim population in Africa. In 2018, the CIA World Factbook estimated that 53.5% of Nigeria's population is Muslim. Islam is predominantly concentrated in the northern half of the country, with a significant Muslim minority in the southern region. Islam was introduced to what is now Nigeria during the 11th century via trade routes with North Africa and the Senegalese basin, and it was the first monotheistic Abrahamic religion to arrive in Nigeria. Christianity was later introduced in the 15th century by Portuguese missionaries, and grew to be a dominant religion alongside Islam. Muslims in Nigeria are predominantly Sunnis of the Maliki school of thought. However, there is a significant Shia minority, primarily in Kaduna, Kano, Katsina, Osun, Kwara, Yobe and Sokoto states (see Shia in Nigeria). In particular, A 2008 Pew Forum survey on religious diversity identified 5% of Nigerian Muslims as Shia. The Ahmadiyya movement also has a sizeable presence in the country. History Islam in Northern Nigeria Islam was introduced to Nigeria during the 11th century through two geographical routes: North Africa and the Senegalese Basin. The origins of Islam in the country is linked with the development of Islam in the wider West Africa. Trade was the major connecting link that brought Islam into Nigeria. Islam was first documented in Central Sudan by medieval Islamic historians and geographers such as Al-Bakri, Yaqut al-Hamawi and Al-Maqrizi and later works of Ibn Battuta and Ibn Khaldun offered more notes about Islam in West Africa. Islam grew in North-East Nigeria, in particular, the Kanem empire as a result of trade between Kanem and Northern African regions of Fezzan, Egypt and Cyrenaica in the eleventh century. Muslim merchants from the North sometimes remained in settlements along trade routes, this merchant class would later preach the message of Islam to their host communities. The first documented conversion of a traditional ruler was in the eleventh century when Mai Ume Jilmi of Kanem was converted by a Muslim scholar whose descendants later held a hereditary title of Chief Imam of Kanem. Writings by Ahmad Fartua an Imam during the period of Idris Alooma provided glimpse of an active Islamic community in Bornu while religious archives showed Islam had been adopted as the religion of the majority of the leading figures in the Borno Empire during the reign of Mai (king)