Influenced by papal bulls, Swedish magnates in the 12th century set up crusading expeditions to convert the heathens in the eastern Baltic. This resulted in the establishment of the Catholic Church, the Christian religion and the Swedish conquest of southern Finland in 1249. Turku, or Åbo, became the principal city in Finland and residence of a Bishopric. As a result of Protestant Reformation in the 16th century the Catholic Church had to give way for the Lutheran state church which was established by King Gustav Vasa of Sweden, whose principal reformer in Finland was Mikael Agricola and from 1554 also the Bishop. After the Finnish War in 1809, Finland became a part of the Russian Empire as an autonomous grand duchy. In 1817 the Bishop was created Archbishop and became head of the Church in Finland, which thereupon became the state church of the grand duchy. In 1870, the church was detached from the state as a separate judicial entity. After Finland had gained independence in 1917, through the constitution of 1919 and the act on religious freedom of 1922 the Evangelical Lutheran Church of Finland became a national church of Finland .. How many years before the Bishop became head of the Church in Finland did Finland became a part of the Russian Empire?