The Chora Museum. This Byzantine Church, which became a mosque (during which time it's astounding mosaics and frescoes were preserved from permanent destruction) and now a museum, is the jewel of Istambul's byzantine relics. What it makes up for in its diminutive size is the magnificence and closeness of its iconography.

For the stories of the bible lessons on its walls and cielings, look up "Chora Museum".