The Aksumite Kingdom, which had its center at Aksum, arose to power in the Tigray Region in the first century AD. It later developed into a significant force on the Red Sea, conquering Yemen and Meroe. Christianity was established as the official religion in the beginning of the fourth century, under the rule of Ezana. The Aksumites initially referred to themselves as "Ethiopians" during Ezana's reign, and soon after, Philostorgius became the first foreign author to refer to the Aksumites as Ethiopians. When Islam gradually replaced trade with the Christian Aksum in the Arabian peninsula, the Aksumite empire began to fall. Aksum's commercial dominance in the area eventually came to an end as it grew isolated and experienced a decline in its economy.