 Succeeding rulers strengthened the Bulgarian state throughout the 8th and 9th centuries . Krum doubled the country 's territory , killed Byzantine emperor Nicephorus I in the Battle of Pliska , and introduced the first written code of law . Paganism was abolished in favour of Eastern Orthodox Christianity under Boris I in 864 . This conversion was followed by a Byzantine recognition of the Bulgarian church and the adoption of the Cyrillic alphabet developed at Preslav which strengthened central authority and helped fuse the Slavs and Bulgars into a unified people . A subsequent cultural golden age began during the 34 @-@ year rule of Simeon the Great , who also achieved the largest territorial expansion of the state . 
