The Christianization of Bulgaria was the process by which 9th-century medieval Bulgaria converted to Christianity. It reflected the need of unity within the religiously divided Bulgarian state as well as the need for equal acceptance on the international stage in Christian Europe. This process was characterized by the shifting political alliances of Boris I of Bulgaria (ruled 852-889) with the kingdom of the East Franks and with the Byzantine Empire, as well as his diplomatic correspondence with the Pope.

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Because of Bulgaria's strategic position, the churches of both Rome and Constantinople each wanted Bulgaria in their sphere of influence. They regarded Christianization as a means of integrating Slavs into their region. After some overtures to each side, the Khan adopted Christianity from Constantinople in 870. As a result, he achieved his goal of gaining an independent Bulgarian national church and having an archbishop appointed to head it.