Kiev (modern Kyiv, Ukraine) (modern Kyiv, Ukraine), served as the capital of the Rus’. It would later be called the mother of Russia. It is also where Russians first converted to Orthodox Christianity. The term Russia seems to originate in the Viking settlers known as the Rus. They mixed with the local Slavic population and created a state centered on Kiev. Byzantines converted them to Christianity, which would have a lasting influence on Russian, Belarusians and Ukrainians. 

The history of Kiev begins under the rule of the Rus. The Rus (also known as Varangians) were original Vikings who began to trade along the Volga River and later the Dnieper River. They establish several principalities centered on cities like Novgorod and later Kiev. Starting in the 9th century, they began to raid and even attack the Byzantine Empire. Vladimir the Great, who ruled the Rus from Kiev, converted to Orthodox Christianity in 988. It is said that he was convinced to convert in part because of his envoy's report about their visit to Constantinople. After visiting Hagia Sophia, they reportedly said "We no longer knew whether we were in heaven or on earth". Certainly the splendor of Constantinople at the time was impressive, and likely influenced Vladimir's conversion. Vladimir was also among the rulers of Kiev who gave military assistance to the Byzantines, leading to the formation of the Varangian Guard.
Once the rulers at Kiev decided to convert, they built churches which were heavily influenced by Byzantine art and architecture. In fact, many Byzantine artists and architects went north to help create Christian structures. This can best be seen in Saint Sophia's Cathedral in Kiev, which dates to the early 11th century. It was designed rival Hagia Sophia in Constantinople, thus symbolizes Kiev as the 'new Constantinople'. There was even a Golden Gate in Kiev, named after the famous Golden Gate of Constantinople. In addition, the Kiev Monastery of the Caves date to 1051 and were influential in the spread of Orthodox thought. Byzantine influence would continue to be felt across the Slavic north, as seen in one of oldest church in Russia, as other Orthodox churches were built, including St. Sophia in Novgorod, dating to the 11th century. Eventually the power of the Rus fragmented, causing a decline in Kiev’s important. This influence can also be seen in several structures of another medieval Russian capital, the city of Vladimir

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Fresco of Kathisma and seating of the Hippodrome

St. Sophia, Kyiv (Google Arts & Culture)