I just got a great prophet from building the Hagia Sophia in one of my cities, but although my religion is not yet enhanced, I can't use my prophet to enhance it. What's going on?
The great prophet has to be in your holy city in order to enhance your religion. Usually they spawn on the capital, but the prophet from the Hagia Sophia spawns in the city in which the wonder was built. Just move it to your holy city and you will be able to enhance your religion.