In c.252, the Sasanian king Shapur I of Persia conquered Armenia. Its king Tiridates II left the country and his children sided with the Persians.note[Zonaras, History 12.21.] The country was ruled by Sasanian princes (Hormizd I, Narseh). Later Armenian traditions, recorded by Agathangelos, maintain that an Armenian leader named Chosroes II continued the war against the Persians but was eventually assassinated.note[Agathangelos, History of St. Gregory and the Conversion of Armenia 1.2-7.]
According to Agathangelos, the son of Chosroes was Tiridates, who fled to the Roman Empire after his father had been killed,note[Agathangelos, History of St. Gregory and the Conversion of Armenia 1.10.] fought in a Roman army,note[Agathangelos, History of St. Gregory and the Conversion of Armenia 1.12-14.] and was made king in Armenia by the Romans.note[Agathangelos, History of St. Gregory and the Conversion of Armenia 1.14.]
Although Agathangelos' subject matter is partly legendary, it does not contradict other information. In 298, the Roman emperor Galerius defeated Narseh (the former governor of Armenia and now king of Sasanian Persia) and appointed a member of the Arsacid family as king of Armenia: Tiridates. He would later accept Christianity. Writing in in the second quarter of the fifth century, Sozomen writes:
Subsequently the Christian religion became known to the neighboring tribes and was very greatly disseminated. The Armenians, I have understood, were the first to embrace Christianity. It is said that Tiridates, then the sovereign of that nation, became a Christian by means of a marvelous Divine sign which was wrought in his own house; and that he issued commands to all the subjects, by a herald, to adopt the same religion.note[Sozomen, History of the Church 2.8.1.]
Agathangelos and Moses of Chorene attribute Tiridates' conversion to the preaching of Gregory the Illuminatornote[Agathangelos, History of St. Gregory and the Conversion of Armenia 4.6; Moses of Chorene, History of the Armenians 2.83.] and it is commonly dated to the first years of the fourth century. This would made the Armenians the first to convert to Christianity, as Sozomen indicates. However, the sequence of his account places it quite late in the reign of Constantine (after Helen's visit to Jerusalem in 326). In any case it is certain that the idea that Armenia was the first Christian kingdom was accepted before the mid-fifth century.
Tiridates was still king in c.330 CE and was succeeded by his son Chosroes III.