The Chaldeans in Iraq were converted to Christianity by St. Thomas the Apostle.

The church is linked to the former Nestorian Church in Iraq, but split from the Nestorians some 450 years ago.

The Chaldeans reunited with Rome in the 5th century and Pope Eugene IV issued a declaration that said all converted Nestorians would henceforth be known as Chaldeans.

The Chaldeans to this day are considered part of the Roman Catholic Church. The Chaldean Church is semi-autonomous, meaning it is affiliated with the Roman Catholic Church, yet retains its own customs, rites, and traditions. The head of the church is based in Baghdad.

The Chaldean population is mainly found in Irbil, Kirkuk, and Mosul. Chaldeans are also found in Lebanon, Syria, Iran, and the United States.