During the Early Viking Age, the people of Sweden were primarily adherents of Norse mythology, like much of southern Scandinavia. Christian influences began to make their way into the region through early contacts during travel and trade, with some burials reflecting Christian practices as early as the late 8th century. Swedish interaction with Irish monks also suggests that Christianity had a presence early on, as some Irish saints were venerated during the Middle Ages.
The earliest significant effort to Christianize Sweden began with the monk Ansgar, who first arrived in Birka in 829, under the Holy Roman Empire's direction. Ansgar built a church and attempted to establish a Christian community, but these efforts faded after his departure. He returned around 850 but found only remnants of his earlier mission. This early attempt to spread Christianity was largely unsuccessful in maintaining a strong, lasting presence. However, archaeological findings, such as Christian graves at Varnhem, show that Christian practices were being adopted by some communities as early as the 9th century.
The full conversion of Sweden took centuries. King Emund the Old, who ascended the throne around 1050, was among the first Christian monarchs. Still, the complete Christianization of Sweden faced resistance, especially from regions like Uppland, where the Temple of Uppsala, dedicated to Norse gods, remained a significant religious center. The last pagan king, Blot-Sweyn, ruled until 1087 when he was killed by his Christian predecessor, King Inge. It wasn't until the reign of King Eric the Saint (mid-12th century) that the Christian Church in Sweden began to organize more formally, with the establishment of the Archbishopric of Uppsala in 1164 marking a major step in consolidating Christian authority in the country.
This slow transition from Norse paganism to Christianity profoundly impacted Swedish society, politics, and culture, laying the groundwork for the eventual dominance of the Christian Church throughout the medieval period.