The Ecclesiastical History of Sozomen

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. I think that the beginning of the conversion of the Persians was owing to their intercourse with the Osroenians and Armenians; for it is likely that they would converse with such Divine men and make experience of their virtue.