Two of the most notable events under Arsacid rule in Armenian history were the conversion of Armenia to Christianity by St. Gregory the Illuminator in 301 and the creation of the Armenian alphabet by Mesrop Mashtots in 405 . Two of the most notable events under Arsacid rule in Armenian history were the conversion of Armenia to Christianity by St. Gregory the Illuminator in 301 AD and the creation of the Armenian alphabet by Mesrop Mashtots in 405 AD .