The Ukrainian Greek Catholic Church is an Eastern Catholic church in full communion with the Pope in Rome and the worldwide Roman Catholic Church. It was originally founded in 988 in Kiev, Ruthenia by Grand Prince Volodymyr the Great, and it was formally founded in 1596 as the merger of the Ruthenian Orthodox Church and the Holy See. The church takes its "Greek" name from its "Byzantine Rite" tradition, while it takes its "Ukrainian" name from its use of the Ukrainian and Church Slavonic languages for church services. By 2019, it was the largest Eastern Catholic church with over 5.5 million believers and 3,993 parishes.