Previous churches have been destroyed, this is the one rebuilt by Emperor Justinian in 536 AD. The Haghia Sophia, Saint Sophia, the church of Holy Wisdom is massive and a little disappointing. It does not retain any elements of its former glory, to me, it is a shell of its former glory, a huge shell but nonetheless, a must see in Istanbul. This is a principal Byzantine building still standing in Islamic Turkey. Even though they say they are secular, Islamic ideas and culture runs very deep and is not lost at all in Turkey. Do not be deceived, the undertones are there and you'll always be the infidel. Tourists from all over the world throng the streets of Istanbul and in some places it gets so crowded that you won't get a chance to see it. Today all the names of churches are 'turkified' and you must use the Turkish name before you'll be understood. The new name for the Haghia Sophia is Aya Sofya. It was used as a mosque for 400 years during the Ottoman reign and one sees Islamic symbols all over. It's quite disconcerting. The glorious mosaics have faded but there is an attempt at restoration. Lots of Byzantine churches have been converted to mosques, for the few that are not, they are called museums. There is a small vestige of Orthodox Christian life in Istanbul, most Christians, who are mostly Greeks have left the country.
I loved this trip because I became interested in the Greek and Armenian genocides perpetrated by the Turkish people under Kemal Ataturk with the complicity of the Germans. One must never forget history, to forget would mean to repeat it.