According to Eastern Orthodoxy, in Slavic countries, Saint Vladimir performed the baptism in the Dnieper River, in 988. He, himself had his own baptism in Constantinople in 987 while on a diplomatic mission. It took about 100 years for Orthodoxy to spread through out the remainder of Kiev Rus, with some cities adopting and some resisting, which resulted in numerous small religious conflicts within the area.

Reference the following web site for a simplified and very brief overview: https://www.britannica.com/topic/Kyivan-Rus