Correct Answer: Vladimir the Great
Vladimir the Great was the Grand Prince of Kiev from 980 until the day he died in 1015. Although he was not the first to introduce Christianity into the Kievan Rus region (today Ukraine and Russia), he was the one who cemented it as the majoritarian religion in the area after his baptism in 988, ordering the end of paganism in the area (not without certain opposition). He unified most of this region unider his mandate.
As for the reason he chose Orthodox Christianity, it is somewhat shrouded in legend. The popular tale says that he sent emissaries to learn about the religions of neighboring. When they came back, he didn't like what he heard about the restrictions Islam and Judaism imposed, and he didn't find anything interesting in what was said of Westen Christianity. Instead, he was marveled by his emissaries tales of the Orthodox temples, as they said "they didn't know whether they were in heaven or Earth". As for the more historically accurate version, it is said that he converted in part as part of a military pact.
