Why is Sudan a Islamic country?

According to official estimates from 2008, Islam comprised about 97% of Sudan's population, with the most followed branch being Sunni Islam. Christianity accounted for the majority of the remaining portion, primarily practiced by Southern Sudanese people who follow various denominations of Christianity, mainly Roman Catholicism and Anglicanism. Other religious minorities including adherents of traditional indigenous beliefs also constitute a small percentage of the population.

Although Sudan experienced several historical periods which shaped its modern day religious identity, its designation as an 'Islamic country' is primarily linked to a referendum held in 2005 that resulted in the partition of the country into Sudan and South Sudan.