The Catholicos of All Armenians leads the Armenian Apostolic Church, the national church of the Armenian people and, by traditional reckoning, one of the oldest state churches in the world. Armenian tradition holds that the apostles Thaddeus and Bartholomew first brought Christianity to Armenia in the first century CE, and that Gregory the Illuminator converted King Tiridates III and the Armenian court around 301 CE, an event Armenian tradition dates earlier than the Roman Empire's own toleration of Christianity under Constantine. Gregory became the church's first Catholicos, and Armenian Christianity has remained closely bound to Armenian national identity through subsequent centuries of foreign rule by Persian, Byzantine, Arab, Mongol, Ottoman and Russian powers.
Like the Coptic Church, the Armenian Apostolic Church did not accept the christological formula of the Council of Chalcedon in 451 CE and is counted among the Oriental Orthodox churches, a family distinct from both Roman Catholicism and Eastern (Chalcedonian) Orthodoxy despite the similarity of names. The church's seat, Etchmiadzin, near Armenia's capital Yerevan, houses what is traditionally regarded as the oldest cathedral in the world, founded in the early fourth century.