The Church of Mor Eliyo at Beth Kustan was constructed in 343 AD. It has been suggested that the village was founded by a member of the Roman
limitanei (frontier militia) named Constans in the 4th century AD. It is believed that the army of the
Roman Emperor Constantine the Great () marched through Beth Kustan on several occasions. The rock of Helen, named after
Helena, mother of Constantine I, is located near the village. Philoxenus Yeshu, metropolitan bishop of the
Monastery of the Cross and
Hah (), ordained by
Ignatius Saba I,
patriarch of Tur Abdin, was from Beth Kustan. Dionysius Yuhanna of Qustan was metropolitan bishop of the Monastery of the Cross in 1519–1543. Cyril Isaiah of Qustan was abbot of the
Monastery of Mar Awgin and then metropolitan bishop of
Nisibin in 1861–1865. In the
Syriac Orthodox patriarchal register of dues of 1870, it was recorded that the village had fifty-four households, who paid one hundred and eleven dues, and was served by the Church of Morī Eliyyō and one priest. In 1914, the village was inhabited by 500 Syriacs, according to the list presented to the
Paris Peace Conference by the Assyro-Chaldean delegation. No Muslims inhabited the village. Upon hearing of the massacres amidst the
Sayfo, the villagers decided to take refuge at Hah and were thus escorted there by Hajo, chief of the Kurtak clan. They stayed at Hah for seven years until they were able to return to Beth Kustan with the help of Çelebi Ağa of the Haverkan confederation. The village was destroyed during the Sayfo and took many years to rebuild and resettle. The village was officially named Alagöz in the 1930s as a result of the state's
turkification policy. The village was transferred from the district of Dargeçit to
Midyat District in 1996. There were less than twenty Syriac families at the village by 2014. On 12 February 2015, Beth Kustan was restored as the official name of the village. The Church of Mor Eliyo was restored by people in the
diaspora. ==Demography==