During Vladimir 's reign in 988 the conversion of the Kievan Rus ' to Christianity took place .
As evidenced by the Rus ' Primary Chronicle , Boris and Gleb were sons of Vladimir I , born to him by the Bulgarian princess .
During Vladimir I 's reign in 988 the conversion of the Kievan Rus ' to Christianity took place .
As evidenced by the Rus ' Primary Chronicle , Boris and Gleb were sons of Vladimir I , born to him by the Bulgarian princess .