The Galatian language, based on onomastic evidence (as no texts written in Galatian have yet been discovered), seems to have closely resembled the Gaulish language of western and central Europe. The language was introduced to Anatolia in the 3rd century BC, when Celtic tribes - notably the Tectosages, Trocmii, and Tolistobogii - migrated south from the Balkans. According to the Greek historian Strabo, the Tectosages of Anatolia were related to the Volcae Tectosages of Gaul; the parent tribe of both branches, the Volcae, originally lived in central Europe.
Contemporary Roman sources
Sometime between AD 48–55, the Apostle Paul wrote his Epistle to the Galatians in Greek, the medium of communication in the eastern parts of the Roman Empire. This may mean that Galatians at the time were already bilingual in Greek, as St. Jerome later reports. However, scholars are divided as to whether Paul was writing to Greek Galatians or to the Hellenized descendants of the Celtic Galatians.
Lucian of Samosata recorded in circa AD 180 that the prophet Alexander of Abonoteichus was able to find Celtic-speaking interpreters for his oracles in Paphlagonia (immediately northeast of Galatia).
The physician Galen of Pergamon in the late 2nd century AD complained that the commonly spoken Greek of his day was being corrupted by borrowings of foreign words from languages such as Galatian.
In the 4th century St. Jerome (Hieronymus) wrote in a comment to Paul the Apostle's Epistle to the Galatians that "apart from the Greek language, which is spoken throughout the entire East, the Galatians have their own language, almost the same as the Treveri". The capital of the Treveri was Trier, where Jerome had settled briefly after studying in Rome.
Survival into Early Medieval period
In the 6th century AD, Cyril of Scythopolis suggested that the language was still being spoken in his own day when he related a story that a monk from Galatia was temporarily possessed by Satan and unable to speak; when he recovered from the "possession", he could respond to the questioning of others only in his native Galatian tongue.