In the circumstances of the linguistic environment of Southeastern Europe, these early phenomena took the stable form of Indo-European dialectal processes. Bulgarians were the bearers of Indo-Iranian cultural behaviour in the main spheres of historical activity. In Europe they encountered a deep Hellenist-Thracian Indo-European linguistic tradition with an enormous vocabulary, which they adopted. On the other hand, the Slavs actually redeveloped the vernacular tongues in terms of the Indo-European element therein, and finally fixed the pre-literary Indo-European aspect of the mixed population, both aboriginal and new.
The unifying role of religion and the introduced Old Bulgarian language accelerate the process of mergence of the ethnic components bulgaria tour which continues also in the 11th century.
With such a start, the ancient Bulgarian language and ancient Bulgarian literature, the work of the holy brothers Cyril and Methodius, made a linguistic progress, unsurpassed in its quality and distinguished by its synthetic character. The Old Bulgarian language and Old Bulgarian books assumed a supra-national significance and usage and also a supra-national state and spiritual and creative functionality for all Slavonic peoples.
The claim that language is not an ethnic defining factor is best supported by the state of the Bulgarian nation during the Middle Ages. Despite the unifying role of religion, the ethnic differences between Slavs and Bulgarians were not completely obliterated in the 10th century. The Old Bulgarian language, which was introduced, enhanced the process of merging the ethnic components.
Yet, in the second half of the 11th century, one can see clearly the differences between the “old” Bulgarians and the “new” ones, who had emerged under the powerful pressure of Christianity and the Slavonic influence during the 9th-10th centuries.
The most impressive literary achievement of the holy brothers Cyril and Methodius from the second half of the 9th century is not the design of the script system. As it is well known, several decades later, the Glagolitic script, which is in fact a new graphic system, created by the holy brothers, was replaced by the Cyrillic alphabet. The real meaning of the work of Cyril and Methodius is the creation of numerous neologisms which were drawn from the lexis of the living speech.
The Old Bulgarian of the holy brothers and their disciples could have easily borrowed the words of the Greek language, ready and shaped by time, and correctly reflecting the complex notions of theological literature. But new words appeared in their stead, building the conceptual framework of the Old Bulgarian idiom. On the other hand, this advantage of the literary Old Bulgarian language of 9th—10th centuries raised it to the level of the two major languages of early medieval Europe – Greek and Latin. Hebrew had a limited use on the continent because it was used in the comparisons of the originals of the Old Testament, although often the Old Greek translation of the Old Testament, made in Alexandria in the 3rd century BC (the so-called Septuaginta or the translation of the 70 scholars) was used. Thus Old Bulgarian became one of the ancient classical languages as early as the 10th century and in the course of time it developed their characteristics. The national literatures of Serbs, Russians and Romanians, which originated on its basis in the Middle Ages, expanded the territory of the Old Bulgarian language in the vast spaces of Southeastern and Eastern Europe.
“Eternal glory to Cyril the Philosopher who translated the word of God from the Greek into Bulgarian language and educated the Bulgarians, the new second apostle in the reign of Mihail and the righteous Queen Theodora, his mother who decorated the divine church with holy icons and championed Orthodoxy”
From Synodic of Tsar Boril. Translated by Ivan Duychev. From Old Bulgarian Literature. Vol. II, Sofia, 1944, p. 167.