The conception of an Ethiopian nation by Ethiopian nationalists is stated to have begun with the Aksumite Kingdom in the 4th century A.D. The Aksumite Kingdom was a predominantly Christian state that at the height of its power controlled what is now the Ethiopian Highlands, Eritrea, and the coastal regions of Southern Arabia. The Aksumite Kingdom was responsible for the development of the religious movement that became the Ethiopian Orthodox Tewahedo Church. However, the expansion of Islam in the 7th century caused the decline of the Aksumite Kingdom, and most of the lowland populations converted to Islam, while the highland people remained Christian. Since the Aksumite people became divided between Christian highlands and Islamic lowlands, religious and tribal tensions and rivalries between the people intensified. The Aksumite society changed into a loose confederation of city-states that maintained the language of Aksum.