As the power of the Lithuanian warlord dukes expanded to the south and east, the cultivated East Slavic Ruthenians exerted influence on the Lithuanian ruling class. They brought with them the Church Slavonic liturgy of the Eastern Orthodox Christian religion, a written language (Chancery Slavonic) that was developed to serve the Lithuanian court's document-producing needs for a few centuries, and a system of laws. By these means, Ruthenians transformed Vilnius into a major center of Kievan Rus' civilization. By the time of Jogaila's acceptance of Catholicism at the Union of Krewo in 1385, many institutions in his realm and members of his family had been to a large extent assimilated already into the Orthodox Christianity and became Russified (in part a result of the deliberate policy of the Gediminid ruling house).