In 301, Armenia became the first nation to adopt Christianity as a state religion, amidst the long-lasting geo-political rivalry over the region. It established a church that today exists independently of both the Catholic and the Eastern Orthodox churches, having become so in 451 after having rejected the Council of Chalcedon. The Armenian Apostolic Church is a part of the Oriental Orthodox communion, not to be confused with the Eastern Orthodox communion. The first Catholicos of the Armenian church was Saint Gregory the Illuminator. Because of his beliefs, he was persecuted by the pagan king of Armenia, and was "punished" by being thrown in Khor Virap, in modern-day Armenia.

He acquired the title of Illuminator, because he illuminated the spirits of Armenians by introducing Christianity to them. Before this, the dominant religion amongst the Armenians was Zoroastrianism. It seems that the Christianisation of Armenia by the Arsacids of Armenia was partly in defiance of the Sassanids.