The Etchmiadzin Cathedral is the Mother Church of the Armenian Apostolic Church and the central building of the Mother See of Holy Etchmiadzin. Located in the center of the city of Vagharshapat (also known as Etchmiadzin, after the cathedral), it is the first church to be built in Armenia. It is also considered the oldest cathedral in the world.
According to the tradition, the original church was built between 301 and 303 by Armenia's patron saint Gregory the Illuminator, following the adoption of Christianity as a state religion by King Tiridates III.
The cathedral is believed to have been built near the royal palace in then Armenian capital city, Vagharshapat, between 301 and 303. According to the tradition, Armenia under King Tiridates III became the first country in the world to adopt Christianity as a state religion in 301. Sources