Answer:
It began when Prince Vladimir of Kiev sent his emissaries to try to find the true religion for his people. After they ventured to many countries and experienced the different faiths, they ended up at the great church of Hagia Sophia in Constantinople during an Orthodox church service. They said the service was so beautiful that they 'did not know whether they were in heaven or on earth, but knew that God was present there.' They reported this to Prince Vladimir and he immediately embraced Orthodox Christianity as the religion for the Russian people in 988 AD.