On the twenty-seventh day of the seventh month, Noah's Ark came to rest on Mount Ararat. When Noah emerged from the Ark, he prepared a sacrifice in the Araratian Valley and built an altar for the sacrifice in the place where Holy Etchmiadzin is now located.
The Etchmiadzin Cathedral was built by St. Gregory the Illuminator, the patron Saint of the Armenian Church. In the solitude of the night, given to spiritual thoughts, St. Gregory had a magnificent vision: The Son of God descending from heaven with a golden hammer in his hand strikes the vastness of the ground, and reveals the place where the Holy Cathedral of Etchmiadzin will be erected.
"I saw an awe-filled human appearance, great and wonderful, who was leading all and from the highest to the lowest, He was before them. He had a great golden hammer in his hand, and all were following Him. He was coming with great speed, flying up like a swift eagle and descending down to the firmament of the earth, in the center of the city, struck the vastness of the land, and an awful wailing resounded up from the chasms of hell."
St. Gregory and the family of the St. Trdat the king, led the people to the place of the Descent of the Only Begotten, and built the Cathedral.
Holy Etchmiadzin is also called "illustrated by light" because it was not designed by earthly architects, but rather the Heavenly Architect, who through a luminous illustration, showed how the new church was to appear.
Construction on the Cathedral of Holy Etchmiadzin concluded in August 303. The consecration of the cathedral occurred on Saturday preceding the Assumption of the Holy Mother of God and was named St. Mary Asdvatsatsin.
As a symbol of the Armenian Church and the faith of the Armenians, the Cathedral of Etchmiadzin, was proclaimed as such shortly after it was built, and instead of being called by her true name, was often simply referred to as "Vagharshapat’s universal", "Universal Church," or merely "First Church."
Hymn for Holy Etchmiadzin
The only begotten of the Father
Descended and the light of glory with Him,
The voices resounded from the depths of Hell.
Having seen the great light, our patriarch Grigor
Told the converted king with joy:
Come let us build the Altar of light,
For there the Light dawned upon us in Armenia.
Author: Catholicos Sahak III of Dzorapor (+703 A.D.)
Source: ArmenianChurch.org