Justinian's Hagia Sophia in Constantinople can be held responsible for the conversion of Pagan Slavs to the Orthodox religion. The emissaries sent to Byzantium were awestruck at the Hagia Sophia's cavernous beauty that mimicked "heaven" with its reflective surfaces and light-filled interior. When they presented this evidence to Vladimir, the leader of Kievan Rus, he decided on this religion over all other religions presented to him at the time.
The origins of Eastern Orthodoxy are steeped in legend, but St. Sophia Cathedral in Kiev is a historical testament to Rus's changing religious face. Even before Peter the Great founded St. Petersburg in an effort to Westernize Russia, Rus's leader was already looking outside of Rus for inspiration during the 10th and 11th centuries.