Armenia is considered the first nation to have adopted Christianity as the state religion, a fact of which Armenians are justifiably proud. The Armenian claim rests on the history of Agathangelos, who states that in 301 A.D., King Trdat III (Tiridates) was baptized and officially Christianized his people. The second, and most famous, state conversion to Christianity was that of Constantine the Great, who dedicated the Eastern Roman Empire in 313 A.D. with the Edict of Milan.
The Armenian Apostolic Church
The Armenian church is known as the Armenian Apostolic Church, so named for the apostles Thaddeus and Bartholomew. Their mission to the East resulted in conversions from 30 A.D. onward, but the Armenian Christians were persecuted by a succession of kings. The last of these was Trdat III, who accepted baptism from St. Gregory the Illuminator. Trdat made Gregory the Catholicos, or head, of the church in Armenia. For this reason, the Armenian Church is sometimes called the Gregorian Church (this appellation is not favored by those within the church).
The Armenian Apostolic Church is part of the Eastern Orthodoxy. It split from Rome and Constantinople in 554 A.D.
The Abyssinian Claim
In 2012, in their book Abyssinian Christianity: The First Christian Nation?, Mario Alexis Portella and Abba Abraham Buruk Woldegaber outline a case for Ethiopia to have been the first Christian nation. First, they cast the Armenian claim into doubt, noting that the baptism of Trdat III was only reported by Agathangelos and over a hundred years after the fact. They also note that the state conversion-a gesture of independence over the neighboring Seleucid Persians-was meaningless to the Armenian population.
Portella and Woldegaber note that an Ethiopian eunuch was baptized shortly after the Resurrection, and was reported by Eusebius. He returned to Abyssinia (then the kingdom of Axum) and spread the faith before the arrival of the apostle Bartholomew. The Ethiopian king Ezana embraced Christianity for himself and decreed it for his kingdom circa 330 A.D. Ethiopia already had a large and strong Christian community. Historical records indicate that his conversion actually happened, and coins with his image bear the symbol of the cross as well.