Boris I, venerated as Saint Boris I (Mihail) the Baptizer, was the ruler (knyaz) of the First Bulgarian Empire in 852–889. The historian Steven Runciman called him one of the greatest persons in history. Despite a number of military setbacks, the reign of Boris I was marked with significant events that shaped Bulgarian and European history. With the Christianization of Bulgaria in 864, paganism was abolished. A skillful diplomat, Boris I successfully exploited the conflict between the Patriarchate of Constantinople and the Papacy to secure an autocephalous Bulgarian Church, thus dealing with the nobility's concerns about Byzantine interference in Bulgaria's internal affairs.

Knyaz or knez, also knjaz, kniaz is a historical Slavic title, used both as a royal and noble title in different times of history and different ancient Slavic lands. It is usually translated into English as prince, depending on specific historical context and the potentially known Latin equivalents of the title for each bearer of the name. These translations probably derive from the fact that the title tsar was often treated as equivalent to "king" by European monarchs. In Latin sources the title is usually translated as princeps, but the word was originally derived from the common Germanic *kuningaz (king).