Page "History of Lithuania" Paragraph 18

As the power of Lithuanian warlord dukes expanded to the south and east, the cultivated East Slavic Ruthenians moved in the opposite direction within their new statehood and exerted influence on the Lithuanian ruling class.

They brought with them the Old Church Slavonic liturgy of the Eastern Orthodox Christian religion, written Slavic language, the version of which known as Chancery Slavonic was to serve the Lithuanian court's document-producing needs for a few centuries, and developed laws, turning Vilnius into a major center of their civilization.

By the time of Jogaila's acceptance of Catholicism at the Union of Krewo in 1385, the establishment of his realm and members of his family had been to a large extent assimilated into the Orthodox Christianity.