 Complete the following phrase:  The Aksumites erected a number of large stelae, which served a religious purpose in pre-Christian times. One of these granite columns is the largest such structure in the world, standing at 90 feet. Under Ezana fl. 320360, Aksum later adopted Christianity. In 615, during the lifetime of Muhammad, the Aksumite King Sahama provided asylum to early Muslims from Mecca fleeing persecution. This journey is known in Islamic history as the 