The Sogdian language belongs to the Eastern subgroup of the Iranian languages. Its core area was Central Asia, in the region surrounding Samarqand and Bukhara, where it was native to the bulk of the population throughout the first millennium AD. For many centuries, Sogdian merchants controlled the central part of the Silk Road and extended their influence far beyond the area of their original habitation. Numerous Buddhist, Manichean, and Christian texts in Sogdian illustrate the religious diversity in Central Asia and western China in the period before the Islamization of the area.

The absolute majority of all Christian texts known to us in Sogdian transmission stems from the ruins of a Christian monastery at Bulayıq in the Silk Road oasis of Turfan (now in Xinjiang, China). Found in situ by the Second German Turfan Expedition in 1905, they are now kept in Berlin as part of the German Turfan Collection. These texts are written in a variety of the Syriac Esṭrangela script, with the addition of three supplementary letters for the phonemes /f/, /χ/, and /ʒ/. This writing system was not used for recording Buddhist or Manichean texts, and therefore it is usually referred to as the Christian Sogdian script. Its orthography reflects quite faithfully the phonetic structure of Late Sogdian and does not display the archaizing features of other Sogdian scripts. The bulk of the available Christian Sogdian manuscripts appear to have been written in the 9th and 10th cent., when Sogdian was likely the lingua franca of the Bulayıq Christian community.

The content of the Bulayıq library indicates that Sogdian Christians belonged to the Ch. of E., which is otherwise known for its missionary activity along the Silk Road. Besides non-denominational compositions, such as Gospel readings, psalms, and lives of early saints, one finds there numerous texts that are particular to Syriac Christianity, such as the life of Yoḥannan of Dailam, a verse homily ‘On the final evil hour’ by Babai of Nisibis, or Dadishoʿ’s Commentaries on Isaiah of Scetis. There is no evidence for indigenous Sogdian Christian literature, while all the identified texts of foreign origin appear, with one exception, to have been translated from Syriac. A large number of Syriac manuscripts have, in fact, been excavated at Bulayıq (see Syriac texts from Turfan), among which are some Sogdian and Syriac bilinguals. In addition, the same site yielded a smaller number of Old Turkic manuscripts, presumably from its terminal phase, and isolated texts in Middle and New Persian. The preparation of the catalogue of texts found at Bulayıq constitutes the main objective of a current SOAS research project The Christian Library from Turfan.

There is no doubt that Bulayıq Sogdian was mutually intelligible with the other known varieties of the Sogdian language. Its peculiar feature, however, is a large number of technical terms of Syriac origin, e.g., ksʾ ‘chalice’, mdbḥʾ ‘altar’, šmšʾ ‘deacon’. The Syriac nouns are normally borrowed in the emphatic state and then inflected as Sogdian ā-stems. By contrast, Buddhist or Manichean Sogdian compositions do not contain any common nouns of undisputed Syriac origin.

No Sogdian texts of Christian content are available from the Sogdian heartland, although Syriac inscriptions are found in the Sogdian archaeological contexts in Urgut (Uzbekistan) and Panjakent (Tajikistan). One Sogdian inscription with Syriac loanwords is found in Kyrgyzstan. The Sogdian finds from the ‘Caves of the Thousand Buddhas’ in the Silk Road Oasis of Dunhuang include a small fragment of a Christian oracle book and several secular texts mentioning local Christians.