The Oldest State-Built Christian Church

The Mother Cathedral of Holy Etchmiadzin (Armenian: Մայր Տաճար Սուրբ Էջմիածին Mayr Tajar Surb Ejmiatsin; originally known as the Holy Mother of God Church, Armenian: Սուրբ Աստուածածին Եկեղեցի Surb Astvatsatsin Yekeghetsi), in the town of Ejmiatsin (Vagharshapat),  Armenia, is the oldest state-built Christian church. It's original vaulted basilica was built in 301-303 by Saint Gregory the Illuminator, who is credited with converting Armenia from paganism to Christianity in 301.

Thus Armenia was the first nation to adopt Christianity as its official religion, preceding Constantine's conversion to Christianity (312-315), and formal adoption of Christianity as the state religion of the Roman Empire in the Edict of Thessalonica (380).