Gregory the Illuminator in the context of "Caesarea (Mazaca)"

The Armenian Apostolic Church (Armenian: Հայ Առաքելական Եկեղեցի, romanized: Hay Aṙaqelakan Ékełetsi) is the autocephalous national church of Armenia. Part of Oriental Orthodoxy, it is one of the most ancient Christian churches. The Armenian Apostolic Church uses the Armenian Rite. The Kingdom of Armenia was the first state in history to adopt Christianity as its official religion (under the Armenian Apostolic traditions) during the rule of King Tiridates III, of the Arsacid dynasty in the early 4th century.

According to tradition, the church originated in the missions of Apostles Bartholomew and Thaddeus (Jude) in the 1st century. St. Gregory the Illuminator was the first official primate of the church. It is sometimes referred to as the Armenian Apostolic Orthodox Church, Armenian Church or Armenian Gregorian Church.

The Arsacid dynasty, called the Arshakuni (Armenian: Արշակունի, romanized: Arshakuni) in Armenian, ruled the Kingdom of Armenia (with some interruptions) from 12 to 428 AD. The dynasty, a branch of the Arsacid dynasty of Parthia, reigned intermittently throughout the chaotic years following the fall of the Artaxiad dynasty until 62, when Tiridates I, brother of Parthian King Vologases I, secured Arsacid rule in Armenia as a client-king of the Roman Empire. However, he did not succeed in establishing his line on the throne, and various princes of different Arsacid lineages ruled until the accession of Vologases II, who succeeded in establishing his own line on the Armenian throne, which ruled the kingdom until the Sasanian Empire abolished it in 428.

Two of the most notable events in Armenian history during the Arsacid period were the conversion of Armenia to Christianity by Gregory the Illuminator and Tiridates III in the early 4th century; and the creation of the Armenian alphabet by Mesrop Mashtots c. 405. In contrast to the more Hellenic-influenced Artaxiads, Arsacid Armenia featured greater Iranian influence.

Etchmiadzin Cathedral is the mother church of the Armenian Apostolic Church, located in the city dually known as Etchmiadzin (Ejmiatsin) and Vagharshapat, Armenia. It is usually considered the first cathedral built in ancient Armenia, and often regarded as the oldest cathedral in the world.

The original church was built in the early fourth century—between 301 and 303 according to tradition—by Armenia's patron saint Gregory the Illuminator, following the adoption of Christianity as a state religion by King Tiridates III. It was built over a pagan temple, symbolizing the conversion from paganism to Christianity. The core of the current building was built in 483/4 by Vahan Mamikonian after the cathedral was severely damaged in a Persian invasion. From its foundation until the second half of the fifth century, Etchmiadzin was the seat of the Catholicos, the supreme head of the Armenian Church.

Isaac or Sahak of Armenia (c. 350 – c. 438) was the catholicos (or patriarch) of the Armenian Church from c. 387 until c. 438. He is sometimes known as Isaac the Great or Sahak the Parthian (Armenian: Սահակ Պարթեւ; Sahak Part῾ew) in reference to his father's Parthian origin. He was the last Armenian patriarch who was directly descended from Gregory the Illuminator, who converted the Kingdom of Armenia to Christianity in the early fourth century and became the first head of the Armenian Church. He supported Mesrop Mashtots in the creation of the Armenian alphabet and personally participated in the translation of the Bible into Armenian.

The Catholicos of All Armenians (Armenian: Ամենայն Հայոց Կաթողիկոս) is the chief bishop and spiritual leader of Armenia's national church, the Armenian Apostolic Church, and the worldwide Armenian diaspora. The Armenian Catholicos (plural Catholicoi) is also known as the Armenian Pontiff (Վեհափառ, Vehapar or Վեհափառ Հայրապետ, Vehapar Hayrapet) 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.

The Armenian Apostolic Church is part of the Oriental Orthodox communion. This communion includes the Coptic Orthodox Church of Alexandria, the Ethiopian Orthodox Tewahedo Church, the Syriac Orthodox Church, the Malankara Orthodox Syrian Church, and the Eritrean Orthodox Tewahedo Church.

Christianity first spread to Armenia prior to the official adoption of the faith in the early fourth century, although the details are obscure. In the early fourth century, the Kingdom of Greater Armenia adopted Christianity as its state religion, becoming the first (or one of the first) state to do so. The Arsacid king of Armenia at the time, Trdat, was converted by Gregory the Illuminator, who became the first head of the Armenian Church. The traditional date for the conversion of Armenia is 301, although alternative dates have been proposed by scholars ranging from 284 to no later than 325. While Armenia's church structure was established at this time, it took longer for Christianity to fully take root in the country. The greatest progress came after the invention of the Armenian alphabet by Mesrop Mashtots and the translation of the Bible and liturgy into Armenian in the fifth century.

The Christianization of Armenia is regarded as one of the most important events in Armenian history, significantly shaping the people's identity, and turning Armenia away from its centuries-long links to the Iranian world. Additionally, the Armenian Church is considered to have provided a structure for the preservation of Armenian identity in the absence of Armenian political independence.

Anak the Parthian (Old Armenian: Անակ Պարթեւ, romanized: Anak Part῾ew; fl. third century) was a Parthian noble who, according to the Armenian tradition, was the father of Gregory the Illuminator, who converted Armenia to Christianity in the early fourth century. Anak is said to have killed King Khosrov of Armenia at the incitement of the Sasanians, leading to his own murder and the extermination of his family, except for the child Gregory, who was saved and taken to Roman territory. The details and historicity of this account have been debated by historians.

Khor Virap (Armenian: Խոր Վիրապ, lit. 'deep dungeon') is an Armenian monastery located in the Ararat Plain in Armenia, near the border with Turkey, about 8 kilometres (5.0 mi) south of Artashat, Ararat Province, within the territory of ancient Artaxata. The monastery was host to a theological seminary and was the residence of the Armenian Catholicos.

Khor Virap's notability as a monastery and pilgrimage site is attributed to the fact that Gregory the Illuminator was initially imprisoned here for 13 years by King Tiridates III of Armenia. Saint Gregory subsequently became the king's religious mentor, and they led the proselytizing activity in the country. In the year 301, Armenia was the first country in the world to be declared a Christian nation. A chapel was initially built in 642 at the site of Khor Virap by Nerses III the Builder as a mark of veneration to Saint Gregory. Over the centuries, it was repeatedly rebuilt. In 1662, the larger chapel known as the "St. Astvatsatsin" (Holy Mother of God) was built around the ruins of the old chapel, the monastery, the refectory and the cells of the monks. Regular church services are held in this church. It is one of the most visited pilgrimage sites in Armenia.