What is the origin of the tradition that Thaddeus founded the Armenian Church? According to tradition, Thaddeus (aka Jude) and Bartholomew were the first disciples to bring the Gospel to Armenia.  As such, they are commemorated as the patron saints of that church.  What is the origin of this tradition?  That is, what is the oldest extant reference to Thaddeus preaching in Armenia?
Fortunately, The Kingdom of Armenia: A History (by M. Chahin) has an online extract that covers this very issue. On page 261, Chahin says tradition has it that King Abgar of Edessa (c. 5 BCE-32 CE) wrote to Jesus begging him to cure him of a malady. The allegedly written reply, along with His portrait (painted by Abgar's messenger, Ananias), purports to inform the king that one of His disciples would visit him, after the Writer's ascension. On the arrival of St. Thaddeus, the king, having been cured, along with his wonderstruck people, was converted to Christianity. Soon after that momentous event, the king died. His nephew, Sanatruk, king of another province in Great Armenia, embraced Christianity when St. Thaddeus visited him, in spite of fear of the wrath of his nobles. His anxiety was justified when he was compelled to apostasise and to martyrise St. Thaddeus.

Chahin goes on to provide historic evidence that there was a substantial Christian community by around 100 CE, although I think this is a typo and Chahin actually meant 200 CE - "for there were persecutions during the reign of Khozrov I (217-52) and his predecessors.  Already by the third and fourth centuries, the throne of the Armenian pontiffs was being called the 'chair of St. Thaddeus'.