In 301, Christianity was proclaimed the state religion in Armenia by St. Gregory the Illuminator. Thus, Armenia became the first country in the world where Christianity became the state religion. However, numerous Armenian and foreign-language sources report on the spread and deep roots of Christianity in Armenia even before 301.

Gregory the Illuminator (poor. Գրիգոր Լուսաւորիչ) became the first Catholicos, that is, the first head of the Armenian Church, after the adoption of Christianity as the state religion. In a visionary foreboding, he is said to have been commissioned by Christ to build the mother church of the Armenians in Wagharshapat, today St. Etchmiadzin. "Echmiadzin" means "the Son of God has descended". Echmiadzin, around twenty kilometers from Yerevan, has been the center of the Armenian Church ever since.

However, the official proclamation of Christianity as the state religion in Armenia by a king did not mean that Christianity would have disappeared from Armenia after the martyrdom of the apostles. The missionary work of the apostles was continued in the following two centuries by other preachers, most of whom had to pay with their lives.

Undoubtedly, the translation of the Bible into Armenian was the crucial practical step in the process of converting Armenia to Christianity. The prerequisite for this step was only created in 406 with the development of the Armenian alphabet by St. Mesrop Maschtotz. The extensive translation movement that followed, actively supported by the Catholicos St. Sahak Bartev and King Vramshapuh, produced an Armenian national literature that was not limited to the most important works of the Church Fathers. This period of cultural and spiritual upheaval in Armenia is referred to in Armenian history as the "Golden Age".

However, the 5th century was also a fateful century. The Armenians had to prove the depth of their Christian faith. This was not the first time and, as history shows us, it will not be the last. The enemy was the Sassanid king Yazkert II (438-457), who wanted to convert Armenia to Mazdaism in addition to his political aspirations. In 451, on the battlefield of Avarayr, General (in Armenian: Sparapet) Vardan Mamikonian (370-451) commanded the Armenian army against a vastly superior army of the Persian king (in Persian: Shah). The general and most of his army had to lay down their lives for the sake of faith and homeland. The Armenian Church commemorates this event every year with a grand celebration called Surb Vardanank.

For the Armenians, the importance of this event lies not primarily in its political or military significance, but rather in the failure of King Yazgert II's plan to convert Armenia to Mazdaism. In reply to the Persian king, who wanted to force them to renounce their Christian faith, they wrote: "No one can turn us from this belief, neither angels nor men, neither sword nor fire, nor water, nor any kind of cruel torture". In his last words before the battle, Vardan Mamikonian said to his comrades: "The enemy assumed that we wear Christianity only as a robe: now he realizes that he cannot change the color of our skin". It is thus clear that the Christian faith had already become an integral part of Armenian social structures by the 5th century.

The Armenian Apostolic Church is autocephalous (self-governing) and belongs to the family of the so-called Oriental Orthodox Churches. This family of churches includes the Coptic, Ethiopian, Eritrean, and Syrian Orthodox Churches, as well as the Indo-Malabar Churches. The Armenian Church has been a member of the World Council of Churches since 1962. It has fellowship with the above-mentioned Oriental Orthodox Churches.

At the head of the church is the Supreme Patriarch and Catholicos of All Armenians, with his official seat in Etchmiadzin. Earlier, on the site of today's Etchmiadzin Cathedral, there was a pagan temple, which played an important role before the introduction of Christianity in the country. When, at the beginning of the 4th century, St. Gregory destroyed the well-known pagan temple of the Sandarament cult and built a church on its ruins, the city was renamed Edchmiadzin and made the seat of the Catholicos. Gregory the Illuminator was the country's first Catholicos. The construction of the church in Echmiadzin is said to have started as early as 301 under the supervision of St. Gregory, and the church was completed in 303.

For historical reasons, in addition to the catholicate of all Armenians, local clerical seats also emerged in the Armenian Church, which were completely independent in their administration and whose heads are also called katholikoi. There is currently one such local catholicate in Lebanon. It is the Catholicate of the House of Cilicia, with its official seat in the Antelias district of Beirut. Its head is Catholicos Aram I, and it is under the jurisdiction of the Middle Eastern bishoprics of Beirut, Aleppo, Damascus and Nicosia.

Besides the Catholics mentioned above, the four hierarchical chairs of the Armenian Church also include the Armenian Patriarchate of Jerusalem since 1311 and the Armenian Patriarchate of Constantinople since 1461.

The National Ecclesiastical Assembly, chaired by the Catholicos, is the highest legislative body of the Church. The secular (about 70 %) and spiritual (about 30 %) delegates of the worldwide Armenian dioceses form this body. Each diocese provides at least one delegate.

The Supreme Spiritual Council (Synod) is the executive body of the Church. In addition to the catholicos, who presides over the council, the council consists of 16 members, half of whom are diocesan bishops and half archbishops respectively. All bishops and archbishops of the church together form the bishops' conference, which deals with theological and canonical questions of the church under the presidency of the catholicos.