Christians live primarily in Baghdad, Basra, Mosul, Erbil, Dohuk, Zakho and Kirkuk and in Assyrian towns and regions such as the Nineveh Plains in the north. Before the advent of Islam, the majority of Iraqis (Mesopotamians) followed Syriac Christianity, Eastern Orthodoxy and Catholicism, Judaism or ancient Mesopotamian religions. There are about 60,000 Iraqi-Armenians who follow either Armenian Orthodox or Armenian Catholics. There are also several thousand Arab Christians who are either Greek Orthodox or Melkite Catholic, and they are largely concentrated in Baghdad. During the period from 2013–2017, with ISIS rapidly sweeping through Iraq's western lands , Christians fled as they feared persecution by the terrorist organisation, as they were to ‘execute’ any person who did not believe in their Sunni sect. Thousands of Iraqi Christians fled to The nation's capital where they found refuge and adequate housing, some of whom have chosen to make Baghdad their new permanent home following the full defeat of ISIS in Iraq. Thousands have also fled to other parts of southern Iraq, such as the Shia-majority city of Najaf which housed thousands of Christians in holy Islamic shrines once they fled from ISIS, which sought to exterminate them. A large population have also returned to their homes en masse following the defeat of ISIS and were able to celebrate Christian festivals of Christmas and Easter in safety with the protection of the NPU and its allies. The majority of the Iraqi Christians belong to the branches of Syriac Christianity whose followers are mostly ethnic Assyrians adhering to both the East Syriac Rite and West Syriac Rite:  Followers of these churches are exclusively ethnic Armenians, using Armenian Rite:  Followers of these churches are an ethnic mix known as Melkites:   Christianity was brought to Iraq in the 1st century by Thomas the Apostle and Mar Addai (Thaddeus of Edessa) and his pupils Aggai and Mari. Thomas and Thaddeus belonged to the twelve Apostles. Iraq's Eastern Aramaic-speaking Assyrian communities are believed to be among the oldest in the world. The Assyrian people adopted Christianity in the 1st century and Assyria in northern Iraq became the centre of Eastern Rite Christianity and Syriac literature from the 1st century until the Middle Ages. Christianity initially lived alongside Mesopotamian religion among the Assyrians, until the latter began to die out during the 4th century. In the early centuries after the Arab Islamic conquest of the 7th century, Assyria (also known as Athura and Assuristan) was dissolved by the Arabs as a geo-political entity, however native Assyrian (known as Ashuriyun by the Arabs) scholars and doctors played an influential role in Iraq.