Final answer:
Kievan Rus' became Christians through the conversion of Prince Vladimir I of Kiev in 988, which led to the mass baptism of his subjects in the Dnieper River in Kiev. The conversion allowed Kievan Rus' to establish trade and diplomatic relations with Byzantium and the Christian kingdoms of the west.
Step-by-step explanation:
Kievan Rus' became Christians through the conversion of Prince Vladimir I of Kiev in 988. Vladimir formed an alliance with the Byzantine emperor and converted from paganism to Christianity. He then baptized his subjects in the Dnieper River in Kiev. The conversion to Orthodox Christianity allowed Kievan Rus' to engage in trade and diplomatic relations with Byzantium and the Christian kingdoms of the west.