The Christianization of Finland was a gradual process that unfolded over several centuries, largely influenced by Sweden and the Catholic Church. Before Christianity, Finland was a land of diverse pagan beliefs, with a mix of shamanistic and nature-based practices.
The spread of Christianity began in earnest during the 12th century, as part of a broader effort by the Catholic Church to Christianize the northern pagan regions. This process was accelerated by the Northern Crusades, particularly the Swedish-led crusades aimed at converting the Finnish population. The First Swedish Crusade, traditionally dated to around 1150 CE and led by King Eric IX of Sweden, is a key event in this narrative. It marked the beginning of Sweden’s efforts to bring Finland under its religious and political influence. Bishop Henry of Uppsala, who accompanied the crusade, became an important figure in Finnish Christian tradition and later a martyr.
During the 13th century, the influence of the Catholic Church grew stronger, and more organized efforts were made to establish the church's presence, especially in southwestern Finland. This period also saw Finland's increasing integration into the Swedish realm, further reinforcing the spread of Christianity. The Second and Third Swedish Crusades in the late 13th century helped consolidate Christian control over regions like Karelia.
By the end of the Middle Ages, Finland was firmly Christian, with the Catholic Church playing a significant role in its cultural and social life. The church established parishes, built stone churches, and aligned Finland more closely with Western Europe. Christianity also helped Sweden strengthen its control over Finland, cementing the religious and political ties between the two regions.