Archaeologists in southeastern Türkiye have determined that a subterranean temple dedicated to the ancient Roman-era belief system of Mithraism at Zerzevan Castle in Diyarbakır was sealed and closed about 1,700 years ago, likely after the spread of Christianity in the region.
The discovery comes from ongoing excavations at the 3,000-year-old fortress, which served as a Roman military settlement. The site sits on a 124-meter-high (407 feet) rocky hill near Demirölçek in the Çınar district, about 13 kilometers (8 miles) from the district center. Researchers say Zerzevan Castle contains the world’s last known Mithraic temple uncovered within a military complex.
Excavations began in 2014 and the site was added to the UNESCO World Heritage Tentative List in 2020.
Archaeologists have uncovered extensive remains across roughly 1,000 acres, including 15-meter-high (49.2 feet) defensive walls stretching 1,200 meters (0.75 miles), a 21-meter (69 feet) watchtower, a church, administrative buildings, residences, grain and weapons storage facilities, rock-cut tombs, water channels and 63 cisterns. The complex also includes an underground church, a 400-person underground shelter, hidden passages and a Mithraic sanctuary used for secret religious rituals.
Researchers say the Mithraic temple appears to have been deliberately closed during the 4th century A.D., when Christianity became dominant in the Roman Empire. The finding is based on both archaeological evidence and a newly deciphered inscription found at the temple entrance in 2017.
The inscription was analyzed over the course of a year by professor Mehmet Sait Toprak of Mardin Artuklu University. Using Syriac and ancient Aramaic epigraphic methods, researchers determined the text reflects writing styles consistent with the 3rd and 4th centuries A.D. Comparisons with Syriac inscriptions from the 2nd and 3rd centuries A.D. in the Şanlıurfa Museum helped confirm the dating.
According to the analysis, the inscription indicates the Mithraic temple was sealed by Christians approximately 1,700 years ago, marking a rare, possibly unique example in the archaeological record of a Mithraic sanctuary being formally closed and “consecrated” in written form.
Project director professor Aytaç Coşkun said the fortress functioned as a key Roman border garrison and that the Mithraic sanctuary was part of its religious and military complex. He said evidence, including coins found at the sealing layers, supports the conclusion that the temple was closed during the Christianization of the Roman Empire.
Coşkun said Mithraism was once widely practiced across the Roman world, particularly in the 2nd and 3rd centuries A.D., but was gradually suppressed or replaced as Christianity spread. In some cases, Mithraic temples were converted into churches, while others, like the one at Zerzevan, were sealed.
“This is an extremely important archaeological discovery,” Coşkun said, adding that the inscription provides answers to long-standing questions about how Mithraic sanctuaries were treated during the transition to Christianity.
Toprak said the inscription contains references that symbolically contrast the Mithraic deity, often described as the “unconquered sun,” with Christian symbolism. He said the text references a “holy cross” and describes the authority of a Christian God, indicating the ritual closure and termination of the temple’s function.
He added that the combination of the inscription and a cross found at the entrance suggests the closure was both administrative and symbolic, marking the formal end of religious activity in the Mithraic sanctuary.
Archaeologists say ongoing excavations and restoration efforts at Zerzevan Castle continue to shed light on Roman frontier life and the religious transformation of the empire’s eastern provinces.
