The correct answer to this open question is the following.
The significance of the adoption of Orthodox Christianity by Kievan Russia was the following.
The Byzantine Empire influence many territories and spread culture, politics, and religious teachings in many regions. Photius, who was a patriarch of the church in the Byzantine Empire, considered that the Rus people from Escandinavia were ready to convert to Orthodox Christianity. That was in 867. But the process was not an easy one because the Rus were pagans. So the significance of the adoption of Orthodox Christianity by Kievan Russia was that it ended up being on different stages until, in the 900s, Vladimir the Great was baptized under the Orthodox Christian religion.