The Arsacid Kingdom of Armenia, itself a branch of the Arsacid dynasty of Parthia, was the first state to adopt Christianity as its religion (it had formerly been adherent to Armenian paganism, which was influenced by Zoroastrianism, while later on adopting a few elements regarding identification of its pantheon with Greco-Roman deities). in the early years of the 4th century, likely AD 301, partly in defiance of the Sassanids it seems. In the late Parthian period, Armenia was a predominantly Zoroastrian-adhering land, but by the Christianisation, previously predominant Zoroastrianism and paganism in Armenia gradually declined. Later on, in order to further strengthen Armenian national identity, Mesrop Mashtots invented the Armenian alphabet, in 405 AD. This event ushered the Golden Age of Armenia, during which many foreign books and manuscripts were translated to Armenian by Mesrop's pupils. Armenia lost its sovereignty again in 428 AD to the rivalling Byzantine and Sassanid Persian empires, until the Muslim conquest of Persia overran also the regions in which Armenians lived.
What religion formerly existed in the Greco-Roman empire in 405 AD?