Cyril and Methodius not only brought Christianity in a common language, they brought Byzantium. The Slavs received a fully articulated system of Christian doctrine and a fully developed Christian civilization. The age of the Seven Ecumenical councils was complete and the doctrines of the Trinity and the Incarnation had already been worked out. Because people were preached to in their own tongue, and of taking services in Slavonic, they truly could make Christianity their own.

Around 864 Patriarch Photius sent a bishop to Kiev(capital of present day Ukraine), but this was stopped by Oleg, who assumed power at Kiev (the chief Rus’ city at this time) in 878. Christian ideas from Byzantium, Bulgaria, and Scandinavia, still came into Kievan-Rus’.

In 954 Princess Olga of Kiev was baptized. This paved the way for what is called the greatest events in the history of the Ukarainian and Russian church, the baptism of Prince Vladimir of Kiev and the Baptism of Rus in 988. Olga’s grandson Vladimir (reigned 980-1015) was converted to Christianity and married Anna, the sister of the Byzantine Emperor. Orthodoxy became the State religion of Rus’, and eventually Russia until 1917. (Rus’ was not completely converted to Christianity at this time, and the Church was at first restricted mainly to the cities, while much of the countryside remained pagan until the fourteenth and fifteenth centuries.)

The 10th and 11th centuries majestic churches and monasteries were built. St. Anthony of the Kiev Caves brought the traditions of Athonite monasticism to Rus’ in 1051.

The Orthodox Church during the Kievan period was subject to Constantinople, and until 1237 the Metropolitans of Rus’ were usually Greek. The Rus’ Church continues to sing in Greek the solemn greeting to a bishop, eis polla eti, despota (“Many years to you, Master”), in memory of the days when the metropolitan came from Constantinople. Most of the rest of the bishops were native Rus. 

Edited excerpt from orthodoxwiki, links to full article, article on the Baptism of Rus and various icons of the Theotokos below…