From the CIA World Factbook:
British influence and control over what would become Nigeria and Africa's most populous country grew through the 19th century. A series of constitutions after World War II granted Nigeria greater autonomy; independence came in 1960. Following nearly 16 years of military rule, a new constitution was adopted in 1999, and a peaceful transition to civilian government was completed. The government continues to face the daunting task of reforming a petroleum-based economy, whose revenues have been squandered through corruption and mismanagement, and institutionalizing democracy. In addition, Nigeria continues to experience longstanding ethnic and religious tensions. Although both the 2003 and 2007 presidential elections were marred by significant irregularities and violence, Nigeria is currently experiencing its longest period of civilian rule since independence. The general elections of April 2007 marked the first civilian-to-civilian transfer of power in the country's history. In January 2010, Nigeria assumed a nonpermanent seat on the UN Security Council for the 2010-11 term.
From Wikipedia:
Nigeria is home to a variety of religions which tend to vary regionally. This situation accentuates regional and ethnic distinctions and has often been seen as a source of sectarian conflict amongst the population.[93] Even though, Nigeria is apparently divided equally between Islam and Christianity between north and south, it is evident that across Nigeria there is widespread belief, albeit suppressed for political reasons, in traditional religious practices.
Based on a World Religious survey (Mapping out the Global Muslim Population) 50.4% of Nigeria's population were Muslims,[94] 50.8% were Christian[95] (15% Protestant, 13.7% Catholic, and 19.6% other Christian), and followers of other religions were 1.4%.[96] The core north is largely Muslim, there are large numbers of both Muslims and Christians in the Middle Belt, including the Federal Capital Territory. In the west of the country, especially in the Yorubaland, the population is said to be evenly divided between Muslims and Christians, while in the southeastern regions are predominantly Christians with widespread traditional beliefs, Catholics, Anglicans, and Methodists are the majority with few traditional beliefs, while the Niger Delta region is mainly Christian.[97]
The majority of Nigerian Muslims are Sunni, but a significant Shia and Sufi minority exists (see Shia in Nigeria) and a small minority of Ahmadiyya. Some northern states have incorporated Sharia law into their previously secular legal systems, which has brought about some controversy.[98] Kano State has sought to incorporate Sharia law into its constitution.[99]
Oil wealth and an even split religiously. They are ripe for another civil war.