Various Christian sects make up a little less than 3 percent of the population. Instability and attacks on the community after 2005 have led to a decline in their numbers. The largest denomination is the Chaldean Church, founded in the fifth century by the followers of the theologian Nestorius. In the sixteenth century they unified with Rome. Centered in Mosul and the surrounding plains, most Chaldeans speak Arabic, although some use a modified version of Syriac as a vernacular.