Hilarion (Russian-Ukrainian Илларион / Іларіон) was born around 1000 in Kievan Rus' — the first post-Vladimirian generation of Slavic Catholic-Christians who had grown up entirely under the baptized Kievan-Catholic-Orthodox order established by St. Vladimir the Great in 988.
He was one of the principal scholars at the court of Grand Prince Yaroslav the Wise (1019–1054) — the golden age of Kievan-Russian Catholic-Orthodox intellectual culture. He served as priest of the royal Berestov church on the outskirts of Kiev, where (per the Russian Primary Chronicle) he had excavated a small cave-hermitage on the Dnieper bluffs for his personal ascetic retreats — the cave that would subsequently become the original cell-hermitage of the Kiev-Pechersk Lavra under Sts. Anthony and Theodosius.
In 1051, in the unprecedented step of ecclesiastical autonomy from the Constantinopolitan Patriarchate, Grand Prince Yaroslav summoned a synod of the Russian episcopate at Kiev which elected Hilarion as Metropolitan of Kiev — making him the first native-Slavic Metropolitan of Kiev (all the previous Metropolitans had been Greek-Byzantine appointees of the Constantinopolitan Patriarchate).
His principal surviving theological work — the 'Слово о Законе и Благодати' (Slovo o Zakone i Blagodati, 'Sermon on Law and Grace') — composed around 1050, probably for the consecration of the Cathedral of St. Sophia at Kiev — is the text of Russian-Slavic Catholic-Orthodox theological literature, the first original Slavic-language theological work after the Cyrillo-Methodian translations, and the principal Russian-language testimony to the Catholic-Christian conversion of the Kievan-Rus' people.
Hilarion's metropolitanate was brief (1051–c. 1055) — after Yaroslav's death in February 1054 (and the coincidental East-West Schism of July 1054), the Constantinopolitan Patriarchate refused to confirm the native-Slavic election and appointed a new Greek Metropolitan; Hilarion either resigned voluntarily or was deposed, retiring (per tradition) to the nascent Kiev-Pechersk Lavra where his original cave-hermitage was being expanded by Sts. Anthony and Theodosius into the principal Russian-Slavic monastic centre. He died at the Kiev-Pechersk Lavra around 1055.