Tiridates III of Armenia

Tiridates III (c. 250s – c. 330), also known as Tiridates the Great or Tiridates IV, was the Armenian Arsacid king from c. 298 to c. 330. In the early 4th century (301, according to tradition) Tiridates adopted Christianity as the state religion of Armenia, thus making the Kingdom of Armenia the first state to officially embrace Christianity.

Source: Wikipedia — https://en.wikipedia.org/wiki/Tiridates_III_of_Armenia