The starting point of the Swedish rule is under a large amount of uncertainty.
It is connected to the efforts of the Catholic Church to expand the faith in the Eastern Baltic Sea region and to Northern Crusades.

According to the legend of Eric the Holy, written in the 1270s, the King of Sweden Eric the Holy and English bishop Henry made the First Crusaded to southwest Finland in the 1150s.
According to the chronicle and other fabulous sources, the bishop Henry was converting people to Christianity in the areas of Finland Proper and Satakunta during the crusade.
The crusade is not considered to be a real event.
Also, the Christianisation of the South-western part of Finland is known to have already started in the 10th century, and in the 12th century, the area was probably almost entirely Christian.
According to Eric Chronicles Swedish kingdom, which was only starting to form, made two crusades to Finland in the 13th century.
The so-called Second Crusade against Tavastians in 1249-1250 and the so-called Third Crusade against Karelians in 1293.
According to historical sources, the reason for the crusades besides spreading catholic faith, were the numerous raids that heathen Finnish tribes made to Sweden.
Pope Alexander IV even accepted marriage between Swedish king Valdemar and Sophia, daughter of Danish king Eric IV, so they could better repel heathen attacks.

By the 14th century, with the successful crusades and the partly colonisation of Finnish coastline with Christian Swedish colonists, what is now Western and Southern Finland and the Karelian isthmus, had become part of Sweden, Catholic Church and the Uppsala diocese.
Eastern Karelia, the Käkisalmi region and the Ladoga Karelia retained their ties to the Orthodox Church and to Novgorod.

The Treaty of Nöteborg, made in 1323 between Sweden and Novgorod, was the first treaty that defined the eastern boundary of the Swedish realm and Finland at least for Karelia.
The boundary in northern Finland remained unclear.
However, Sweden annexed the Finnish population on the shores of Northern Ostrobothnia in the 14th century to its realm.
The starting point of the Swedish rule is under a large amount of uncertainty. It is connected to the efforts of the Catholic Church to expand the faith in the Eastern Baltic Sea region and to Northern Crusades.
According to the legend of Eric the Holy, written in the 1270s, the King of Sweden Eric the Holy and English bishop Henry made the First Crusaded to southwest Finland in the 1150s. According to the chronicle and other fabulous sources, the bishop Henry was converting people to Christianity in the areas of Finland Proper and Satakunta during the crusade. The crusade is not considered to be a real event. Also, the Christianisation of the South-western part of Finland is known to have already started in the 10th century, and in the 12th century, the area was probably almost entirely Christian. According to Eric Chronicles Swedish kingdom, which was only starting to form, made two crusades to Finland in the 13th century. The so-called Second Crusade against Tavastians in 1249-1250 and the so-called Third Crusade against Karelians in 1293. According to historical sources, the reason for the crusades besides spreading catholic faith, were the numerous raids that heathen Finnish tribes made to Sweden. Pope Alexander IV even accepted marriage between Swedish king Valdemar and Sophia, daughter of Danish king Eric IV, so they could better repel heathen attacks.
By the 14th century, with the successful crusades and the partly colonisation of Finnish coastline with Christian Swedish colonists, what is now Western and Southern Finland and the Karelian isthmus, had become part of Sweden, Catholic Church and the Uppsala diocese. Eastern Karelia, the Käkisalmi region and the Ladoga Karelia retained their ties to the Orthodox Church and to Novgorod.
The Treaty of Nöteborg, made in 1323 between Sweden and Novgorod, was the first treaty that defined the eastern boundary of the Swedish realm and Finland at least for Karelia. The boundary in northern Finland remained unclear. However, Sweden annexed the Finnish population on the shores of Northern Ostrobothnia in the 14th century to its realm.
Finland under Swedish rule