Acilisene, the ancient city that is now Erzincan, was the site of the Peace of Acilisene by which in AD 387 Armenia was divided into two vassal states, a smaller one dependent on the Byzantine Empire and a larger one dependent on Persia.
This is the name (Ἀκιλισηνή in Greek) by which it is called by Strabo in his Geography, 11.4.14.
The etymological origin of the word is disputed, but it is agreed that the city was once called Erez.
For a while it was called Justinianopolis in honour of Emperor Justinian.
In more recent Greek it has been called as Κελτζηνή (Keltzene) and Κελεζηνή (Kelezene)
In the Armenian language, the 5th-century Life of Mashtots called it Yekeghiats In the more recent past, it was known in Armenian as Երզնկա (Yerznka)
In the settlement of Erez, at a yet unidentified site, there was a pre-Christian shrine dedicated to the Armenian goddess Anahit.
A text of Agathangelos reports that during the first year of his reign, King Trdat of Armenia went to Erez and visited Anahit's temple to offer sacrifice.
He ordered Gregory the Illuminator, who was secretly a Christian, to make an offering at its altar.
When Gregory refused, he was taken captive and tortured, starting the events that would end with Trdat's conversion to Christianity some 14 years later.
After that conversion, during the Christianisation of Armenia, the temple at Erez was destroyed and its property and lands were given to Gregory.
It later became known for its extensive monasteries.

It is hard to tell when Acilisene became a bishopric.
The first whose name is known is of the mid-5th century: Ioannes, who in 459 signed the decree of Patriarch Gennadius I of Constantinople against the simoniacs.
Georgius or Gregorius (both forms are found) was one of the Fathers of the Second Council of Constantinople (553), appearing as "bishop of Justinianopolis".
Theodorus was at the Third Council of Constantinople in 681, signing as "bishop of Justinianopolis or the region of Ecclenzine".
Georgius was at the Photian Council of Constantinople (879).
Until the 10th century, the diocese itself appears in none of the Notitiae Episcopatuum.
At the end of that century, they present it as an autocephalous archdiocese, and those of the 11th century present it as a metropolitan see with 21 suffragans.
This was the time of greatest splendour of Acilisene, which ended with the decisive defeat of the Byzantines by the Seljuq Turks at the Battle of Manzikert in 1071.
After the 13th century, there is no mention of diocesan bishops of Acilisene and the see no longer appears in Notitiae Episcopatuum.
No longer a residential bishopric, Acilisene is today listed by the Catholic Church as a titular see.

In 1071 Erzincan was absorbed into the Mengüçoğlu under the Seljuk Sulëiman Kutalmish.
Marco Polo, who wrote about his visit to Erzincan, said that the "people of the country are Armenians" and that Erzincan was the "noblest of cities" which contained the See of an Archbishop.
In 1243 it was destroyed in fighting between the Seljuks under Kaykhusraw II and the Mongols.
However, by 1254 its population had recovered enough that William of Rubruck was able to say an earthquake had killed more than 10,000 people.
During this period, the city reached a level of semi-independence under the rule of Armenian princes.

Erzincan was one of the most pivotal towns in Safavid history.
It was there, in the summer of 1500, that about 7,000 Qizilbash forces, consisting of the Ustaclu, Shamlu, Rumlu, Tekelu, Zhulkadir, Afshar, Qajar and Varsak tribes, responded to the invitation of Ismail I, who would aid in him establishing his dynasty.
Acilisene, the ancient city that is now Erzincan, was the site of the Peace of Acilisene by which in AD 387 Armenia was divided into two vassal states, a smaller one dependent on the Byzantine Empire and a larger one dependent on Persia. This is the name (Ἀκιλισηνή in Greek) by which it is called by Strabo in his Geography, 11.4.14. The etymological origin of the word is disputed, but it is agreed that the city was once called Erez. For a while it was called Justinianopolis in honour of Emperor Justinian. In more recent Greek it has been called as Κελτζηνή (Keltzene) and Κελεζηνή (Kelezene)
In the Armenian language, the 5th-century Life of Mashtots called it Yekeghiats In the more recent past, it was known in Armenian as Երզնկա (Yerznka)
In the settlement of Erez, at a yet unidentified site, there was a pre-Christian shrine dedicated to the Armenian goddess Anahit. A text of Agathangelos reports that during the first year of his reign, King Trdat of Armenia went to Erez and visited Anahit's temple to offer sacrifice. He ordered Gregory the Illuminator, who was secretly a Christian, to make an offering at its altar. When Gregory refused, he was taken captive and tortured, starting the events that would end with Trdat's conversion to Christianity some 14 years later. After that conversion, during the Christianisation of Armenia, the temple at Erez was destroyed and its property and lands were given to Gregory. It later became known for its extensive monasteries.
It is hard to tell when Acilisene became a bishopric. The first whose name is known is of the mid-5th century: Ioannes, who in 459 signed the decree of Patriarch Gennadius I of Constantinople against the simoniacs. Georgius or Gregorius (both forms are found) was one of the Fathers of the Second Council of Constantinople (553), appearing as "bishop of Justinianopolis". Theodorus was at the Third Council of Constantinople in 681, signing as "bishop of Justinianopolis or the region of Ecclenzine". Georgius was at the Photian Council of Constantinople (879). Until the 10th century, the diocese itself appears in none of the Notitiae Episcopatuum. At the end of that century, they present it as an autocephalous archdiocese, and those of the 11th century present it as a metropolitan see with 21 suffragans. This was the time of greatest splendour of Acilisene, which ended with the decisive defeat of the Byzantines by the Seljuq Turks at the Battle of Manzikert in 1071. After the 13th century, there is no mention of diocesan bishops of Acilisene and the see no longer appears in Notitiae Episcopatuum. No longer a residential bishopric, Acilisene is today listed by the Catholic Church as a titular see.
In 1071 Erzincan was absorbed into the Mengüçoğlu under the Seljuk Sulëiman Kutalmish. Marco Polo, who wrote about his visit to Erzincan, said that the "people of the country are Armenians" and that Erzincan was the "noblest of cities" which contained the See of an Archbishop. In 1243 it was destroyed in fighting between the Seljuks under Kaykhusraw II and the Mongols. However, by 1254 its population had recovered enough that William of Rubruck was able to say an earthquake had killed more than 10,000 people. During this period, the city reached a level of semi-independence under the rule of Armenian princes.
Erzincan was one of the most pivotal towns in Safavid history. It was there, in the summer of 1500, that about 7,000 Qizilbash forces, consisting of the Ustaclu, Shamlu, Rumlu, Tekelu, Zhulkadir, Afshar, Qajar and Varsak tribes, responded to the invitation of Ismail I, who would aid in him establishing his dynasty.
Erzincan