Armenia was the first country to adopt Christianity as its official religion , when St. Gregory the Illuminator converted Tiridates III ( the King of Armenia ) and members of his court , traditionally dated to 301 ( after Mikayel Chamchian 1784 ) . The Kingdom of Armenia was the first state to adopt Christianity as its religion when St. Gregory the Illuminator converted King Tiridates III and members of his court , an event dated to AD 301 .