The Assyrian Church of the East is an ancient apostolic Christian church rooted in Mesopotamia and Persia, tracing its founding to the apostles Thomas, Thaddeus and Bartholomew and its historic center to the See of Babylon at Seleucia-Ctesiphon. Represented already at the Council of Nicaea in 325, the church separated from the churches within the Roman Empire after the Council of Ephesus in 431, and its missionaries subsequently carried the faith east along the Silk Road, establishing communities that at their furthest reach stretched to Xi'an in China and Kerala in India, making it for centuries one of the most geographically extensive Christian bodies in the world.
That reach was reversed by successive catastrophes: Mongol era upheaval, Timurid conquest, and persecution culminating in massacres during and after the First World War, particularly the Sayfo, the Assyrian genocide carried out alongside the Armenian genocide, which devastated the church's heartland communities in what is now southeastern Turkey, northern Iraq and northwestern Iran. The church that survives today is a fraction of its medieval extent, centered in Iraq and sustained by a global diaspora scattered across the Americas, Europe and Australia by successive waves of modern displacement.
Christoph Baumer's illustrated history of the church situates this trajectory, from apostolic origin through medieval expansion to modern near destruction and diaspora survival, as one of the least known major narratives in world Christian history, obscured in part by the church's small modern size and in part by decades of scholarly focus on Western and Byzantine Christianity at the expense of the churches that developed east of the Roman world.