The Bulgarians are of Slavonic origin, and their race is among the oldest in Europe. In the latter part of the fifth century they crossed the Danube, and gave their name to the country between that river and the Balkan Mountains. In subsequent ages they extended their conquests into Thrace and Macedonia, and, encamping before the walls of Constantinople, sought to drive the Byzantine emperors into Asia Minor. In 712, the Bulgarian troops defeated the armies of the Eastern Roman Empire, and laid siege to Constantinople. Three years later their king concluded a commercial treaty with the Emperor Theodosius III. which is said to have remained in force for a long time. In the year 814 the Bulgarians again invaded the Roman Empire, captured Adrianople, and carried a bishop named Manuel, with others of the citizens, into captivity. This person formed the companions of his captivity into a church, and they remained true to their faith, and labored earnestly for its spread. Having made proselytes among the Bulgarians, the bishop and many of the captives suffered martyrdom. Somewhat later, a captive monk, named Constantine Cypharas, endeavored to carry forward the work thus commenced; but the Greek empress, Theodora, for some special reasons, was led to redeem this monk, and procure his return to his native country. At this juncture, a sister of the Bulgarian king Bogoris was residing at Constantinople, whither she had been conveyed as a captive in early youth, and where she had been educated as a Christian, and the effort to secure the return of the monk resulted in her being sent back to her friends. She now labored to gain over the king, her brother, to the Christian faith. Circumstances at length favored her pious efforts, and she sent for Methodius of Thessalonica, a monk and a skilful painter. He was afterwards joined by his older brother Constantine, or Cyrill, surnamed the Philosopher, on account of his learning. Cyrill reduced the Slavonic language to writing, taught the barbarous nation the use of letters, and translated the Scriptures into that language. In the year 861 he baptized king Bogoris. The king undertook to force his people to change their religion and they revolted. He succeeded in suppressing the rebellion, and showed the superficial nature of his Christianity by the cruel revenge he took on the leaders of the revolt. Then the nation followed the lead of their king, and has ever since been nominally Christian. Neander says, that Cyrill was distinguished from all other missionaries of that period, by not yielding to the prejudice which regarded the languages of the rude nations as too profane to be employed for sacred uses, and by not shrinking from any toil which was necessary to master the language of the people among whom he labored.