but byzantium was ultimately conquered. muslim armies arrived and converted it into a mosque. five years later it was changed again turned into a museum. and today is one of turkey's most popular tourist destinations. but turkey is changing. a symbol of the deep struggle between those who want turkey to remain secular and others who want to revive its islamic past. and for now it's the islamists who have the upper hand. alid wants to turn back the tide of history. he leads a movement to turn it back into a mosque and plans to try and block the pope from entering. a mosque owned by muslims, he says. and if we don't reclaim it, we will be punished by god. already his supporters in the tens of thousands gather in front of aya sofia to pray. the islamists have a powerful ally, turkey's president, bishop erdogan. he's resisted calls to make it a mosque but allows prayers outside to go ahead. >> it's the best choice because it's the monumental work of humanity. >> reporter: a monument that embodies nearly two millennia of religious power struggles that have returned to the shores. r