The Catholicos of All Armenians is the chief bishop and spiritual leader of Armenia's national church, the Armenian Apostolic Church, and the worldwide Armenian diaspora. The Armenian Catholicos is also known as the Armenian Pontiff and by other titles. According to tradition, the apostles Saint Thaddeus and Saint Bartholomew brought Christianity to Armenia in the first century. Saint Gregory the Illuminator became the first Catholicos of All Armenians following the nation's adoption of Christianity as its state religion in 301 AD. The seat of the Catholicos, and the spiritual and administrative headquarters of the Armenian Church, is the Mother See of Holy Etchmiadzin, located in the city of Vagharshapat.