The influence of Cyril and Methodius upon the Bulgarian language and culture is incalculable. They not only carried a new liturgical form to Bulgaria but also devised a new alphabet known as Cyrillic. This new alphabet soon replaced Latin and Greek as the only form of writing, and on its base a new Slavic literature and culture grew up.
When Bulgaria adopted Christianity from Byzantium, it also adopted Byzantium's territorial ambitions. Under Tsar Simeon (A.D. 893-927), a period known as the Golden Age, Bulgaria extended its territories from the Black Sea in the east to the southern Carpathian Mountains in the north, to the Sava River in the west, and to Macedonia in the southwest. It was in this period that Bulgaria reached the peak of its territorial expansion, penetrating deep into the Byzantine Empire. Macedonia and Albania became Bulgaria's new frontiers; in 924 Serbia fell under Bulgarian rule. With these victories Simeon claimed the title tsar of all the Bulgarians and the Greeks.
With the territorial expansion came a domestic flourishing in the arts and an increase in trade. The arts and architecture of the period were significant for their beauty and vitality. Preslav, then the capital city, became the center of culture. Crafts, such as goldsmithing, pottery, stonemasonry, and blacksmithing grew, and shops sprang up everywhere. At the same time literature flourished, and education and scholarship took on a new importance. Knowledge of Slavic literature became widespread, and writers treated such varied topics as religion, grammar, logic, and patriotism.
By the end of the tenth century A.D., however, the First Bulgarian Kingdom was beginning to decline. Internally, the local population was weary from continual warring and from the oppression of feudalism. The boyars continued to struggle against the king and his council for their own autonomy. Because of the internal weakness of the country, Bulgaria's neighbors began to encroach on her borders. The Magyars (Hungarians) attacked from the northwest, seizing territory north of the Danube River. The Byzantines in 967 formed an alliance with the prince of Kiev in Russia and, because of this alliance, succeeded in invading Bulgaria repeatedly.