The Armenian Church believes in apostolic succession through the apostles Bartholomew and Thaddeus. According to legend, the latter of the two apostles is said to have cured Abgar V of Edessa of leprosy with the Image of Edessa, leading to his conversion in 30 AD. Thaddaeus was then commissioned by Abgar to proselytize throughout Armenia, where he converted the king Sanatruk's daughter, who was eventually martyred alongside Thaddeus when Sanatruk later fell into apostasy. After this, Bartholomew came to Armenia, bringing a portrait of the Virgin Mary, which he placed in a nunnery he founded over a former temple of Anahit. Bartholomew then converted the sister of Sanatruk, who once again martyred a female relative and the apostle who converted her. Both apostles ordained native bishops before their execution, and some other Armenians had been ordained outside of Armenia by James the Just. Scholars including Bart Ehrman,
Han Drijvers, and
W. Bauer dismiss the conversion of Abgar V as fiction.
According to Eusebius and Tertullian, Armenian Christians were persecuted by kings Axidares, Khosrov I, and Tiridates III, the last of whom was converted to Christianity by Gregory the Illuminator. Ancient Armenia's adoption of Christianity as a state religion (the first state to do so) has been referred to by
Nina Garsoïan as "probably the most crucial step in its history." This conversion distinguished it from its Iranian and Mazdean roots and protected it from further Parthian influence. Other scholars as well have stated that the acceptance of Christianity by the Arsacid-Armenian rulers was partly in defiance of the Sassanids.
When King Tiridates III made Christianity the state religion of Armenia between 301 and 314, it was not an entirely new religion there. It had penetrated the country from at least the third century, and may have been present even earlier.
Tiridates declared Gregory to be the first Catholicos of the Armenian Church and sent him to Caesarea to be consecrated. Upon his return, Gregory tore down shrines to idols, built churches and monasteries, and ordained many priests and bishops. While meditating in the old capital city of Vagharshapat, Gregory had a vision of Christ descending to the earth and striking it with a hammer. From that spot arose a great Christian temple with a huge cross. He was convinced that God intended him to build the main Armenian church there. With the king's help he did so in accordance with his vision, renaming the city Etchmiadzin, which means "the place of the descent of the Only-Begotten".
Initially, the Armenian Church participated in the larger Christian world and its Catholicos was represented at the First Council of Nicea (325). In 353, King Papas (Pap) appointed Catholicos Husik without first sending him to Caesarea for commissioning before Rome had any plans for a universal Roman church. Its Catholicos was still represented at the First Council of Constantinople (381).
Christianity was strengthened in Armenia in the 5th century by the translation of the Bible into the Armenian language by the native theologian, monk, and scholar, Saint Mesrop Mashtots. Before the 5th century, Armenians had a spoken language, but it was not written. Thus, the Bible and Liturgy were written in Greek or Syriac rather than Armenian. The Catholicos Sahak commissioned Mesrop to create the Armenian alphabet, which he completed in 406. Subsequently, the Bible and Liturgy were translated into Armenian and written in the new script. The translation of the Bible, along with works of history, literature and philosophy, caused a flowering of Armenian literature and a broader cultural renaissance.
Although unable to attend the Council of Ephesus (431), Catholicos Isaac Parthiev sent a message agreeing with its decisions. However, non doctrinal elements in the Council of Chalcedon caused certain problems to arise.
At the First Council of Dvin in 506 the synod of the Armenian, Georgian, and Caucasian Albanian bishops were assembled during the reign of Catholicos Babken I. The participation of the Catholicoi of Georgia and Albania were set to make clear the position of the churches concerning the Council of Chalcedon. The "Book of Epistles" mentions that 20 bishops, 14 laymen, and many nakharars (rulers of Armenia) participated in the council. The involvement in the council discussion of different level of lay persons seemed to be a general rule in Armenia.
Almost a century later (609–610) the 3rd Council of Dvin was convened during the reign of Catholicos
Abraham I of Aghbatank and Prince
Smbat Bagratuni, with clergymen and laymen participating. The Georgian Church disagreed with the Armenian Church having approved the christology of Chalcedon. This council was convened to clarify the relationship between the Armenian and Georgian churches. After the Council, Catholicos Abraham wrote an encyclical letter addressed to the people, blaming Kurion and his adherents for the schism. The Council never set up canons; it only deprived Georgians from taking Communion in the Armenian Church. Despite this, the Albanian Church remained under the jurisdiction of the Armenian Church while in communion with the Georgian Church.