Vladimir was a grandson of St. Olga of Kiev who became Grand Prince of Kiev in 980 after a brutal succession war against his brothers. The early years of his reign saw aggressive paganism — he set up idols of Perun and the Slavic gods on a hill above Kiev. But influenced by his grandmother's memory and by careful diplomatic deliberation, around 987 he undertook the search for a new religion for the Rus' that the Russian Primary Chronicle records in famous detail.
He sent envoys to investigate Islam, Judaism, Western (Latin) Christianity, and Byzantine (Greek) Christianity. The envoys to Constantinople, attending the divine liturgy at Hagia Sophia, reported back: 'We did not know whether we were in heaven or on earth.' Vladimir resolved on Byzantine Christianity. After negotiating a marriage with the Byzantine Princess Anna and a Byzantine baptism in Crimea, he returned to Kiev and on a single day in 988 — by the Chronicle's account — ordered the people of Kiev into the river Dnieper and baptized them en masse.
From that day Russian-Ukrainian Christianity was born. Vladimir built the great Tithe Church (the first stone church in the Rus'), founded monasteries on the Greek model, married Anna Porphyrogenita to secure the Byzantine connection, and devoted his remaining twenty-seven years to entrenching the new faith. He died at Berestove in 1015 — the Eastern Orthodox, Catholic, and Russian Orthodox calendars all hail him as Equal-to-the-Apostles.