Perun was the top god of the Slavic pantheon, a sky god with the power of thunder and lightning and storms. He was god of war and law, fertility and oak trees, and his symbols were the eagle and the hammer or axe (many "Axes of Perun" amulets have been found in archaeological digs). Perun was important to many peoples in the region, and even to Vladimir's own military.
So when Vladimir commanded that everyone come to the Dnieper to be baptized in the river or be named his enemy, people were upset, and many resisted. Dobrynya, Vladimir's uncle—Novgorod chronicles claim that it was he who raised Vladimir as a child, and later persuaded him to take control from his older brother—is said to have driven Novgorod to Christianity "by fire."
Pagan protests took place in areas outside of main population centers, such as in the Upper Volga and in the northeast in what is now Rostov in the Yaroslavl Oblast in Russia. Even decades later, in 1071, the bishop was being threatened by a mob opposed to Christianity, but the local prince bisected a "sorcerer" with an axe, saving the bishop and discouraging the uprising. Pagan culture could not be eradicated completely, surviving for centuries: a medieval epic poem composed probably shortly after 1200 called The Tale of Igor's Campaign blends Christian motifs with pagan gods.
Mass conversion was a political move, but it did not immediately change the hearts and minds of the subjects or the culture in win they lived.
But since we've touched on Slavic literature, I want to talk more about Vladimir's uncle Dobrynya, and the Kievan version of the Round Table. More on that tomorrow.