Bulgaria's place in religious history is not principally about Bulgaria. Boris I accepted Christianity in 864 and then did something with longer consequences than the conversion itself: he received the disciples of Cyril and Methodius after they were expelled from Moravia, and gave them the institutional backing to build a Slavonic literary church. The schools at Preslav and Ohrid produced the Cyrillic alphabet, refined the translation of the liturgy and the scriptures into Old Church Slavonic, and made it a third liturgical language of Christendom alongside Greek and Latin. Russia, Serbia, Ukraine and Macedonia inherited that, which is why a decision taken in a ninth century Bulgarian court is legible on a Moscow church wall.
The other durable Bulgarian contribution is heretical. Bogomilism, which appeared here in the tenth century, taught a dualist cosmology in which the visible world is the work of a lesser power, rejected the sacraments and the hierarchy, and spread west along trade routes into the Balkans and eventually into Languedoc, where the connection to Catharism is argued about but not dismissed.
The constitution names Eastern Orthodoxy the traditional religion of Bulgaria without establishing it. Five centuries of Ottoman rule left a Muslim population of roughly a tenth, made up of ethnic Turks and of the Bulgarian speaking Pomaks. The assimilation campaign of the 1980s, which forced Slavic names on them and closed mosques, drove several hundred thousand people to Turkey and is a live memory rather than a historical one.