The popular image of Norse/Christian relations is, in general, wildly inaccurate. Pop culture such as History's Vikings gives us an incredibly skewed view of how the Norse interacted with their The idea of resolutely pagan warriors sailing up to defenseless Christian settlements and slaughtering them all and plundering whatever isn't nailed down holds a certain romantic appeal, but it is not the whole truth by a longshot. Christianity made inroads into Scandinavia relatively quickly and before long the modern countries of Norway, Denmark, Sweden, and Iceland were converted. The process from "official" (usually a king) conversion to the last remnants of paganism disappearing lasted a little over 200 years. Indeed much of the Scandinavian world received Christianity enthusiastically and quickly.

Christianity first made inroads to Scandinavia from missionaries in the 8th century but these seem to have had limited success. While there is some archaeological evidence indicating some Christian populations in Scandinavia at this time, they were still small and received little institutional support from the broader Christian world or from their rulers.

However this started to change in the various Scandinavian polities starting in the 10th century. Harald Bluetooth, after seemingly attempting to craft a conspicuously antique version of paganism (as evidenced by his resurrection of barrow building traditions according to Anders Winroth) instead opted for conversion to Christianity. The direct cause behind this is obscure. Adam of Bremen claims that it was due to his defeat in battle by the Roman emperor Otto. Widukind of Corvey however claims that Harald was converted by a visiting bishop who performed a miracle through carrying a hot iron without being burned. Regardless of the direct reason, Harald was the first Norse ruler to adopt Christianity and the other leading figures of Denmark and Norway quickly followed suit.

Norway specifically is a bit of a more difficult case to examine. Myths and legends (surviving in the Sagas) surrounding early kings of Norway complicate the matter as some of these figures may have not actually even existed, such as Haakon the Good, supposedly fostered by the King of England and made a Christian. Harald Bluetooth claimed rulership over Norway, and in his Jelling stone inscription claims to have Christianized both countries, but especially in Norway's case it seems that much of the actual legwork of conversion was undertaken by his eventual successor in Norway, St. Olaf (or King Olav II). There were other kings between these two but their efforts at Christianization are a little muddled.

Iceland adopted Christianity officially following a vote in the Thing in 1000 and while at first pagan practices were tolerated so long as they were practiced in secret, in practice however it seems that these customs disappeared rather quickly.

Sweden took the longest to convert to Christianity and it took several martyred missionaries before it seems that conversion took in Sweden, but it followed a similar pattern as in the other Nordic countries once it did.

So how did conversion work on a practical level? We are after all in a time before the uniform practices that we associate with Catholicism really came to exist, and the practices and beliefs of the pre-conversion Norse were varied. We need to understand that Norse religious practices were likely not very similar to our own modern conceptions of religion. Belief was not really a key factor in honoring the Norse gods, instead acts of devotion such as sacrifices were the source of engagement with the divine.

There are also some surviving legislation that was used to aid along conversion. Practices such as horse consumption and infant exposure were prohibited, the legal status of clergy was raised to that of nobles, and in some extreme cases resolute pagans were the subject of state violence such as executions and massacres.

Other methods were also employed. Famously the Pope Gregory the Great, during the conversion of England, had suggested that pagan practices be accommodated as much as possible in the creation of new Christian communities with festivals and sacred spaces rededicated to Christ or the Virgin, however it is unclear how closely this was adhered to in practice. Many examples of supposed syncretic practices that you see online are not actually authentic to the Medieval period and are later constructions.

The top down method of conversion in Scandinavia was particularly important as the local rulers and even more powerful officials seem to have been largely in charge of the pagan practices. The Norse religious tradition lacked organized clergy and instead local bigwigs undertook sacrifices, festivals, and so on. Once these men started to convert to Christianity, the overtly pagan aspects of these festivals faded away or were actively suppressed.

Christianity was also quite attractive as it offered prestige and greater economic opportunity for converts, as well as practical benefits for newly Christian realms through the Church's administration system. Literacy in the Latin script brought alongside it the institutional experience of the Church, and this was a boon for the centralization of power in Scandinavia. The prestige attached to the religion cannot be underestimated. Norsemen traveling abroad would scarcely have seen the monumental structures of Christian churches such as the Hagia Sophia in Constantinople, and even less grandiose structures in England, Francia, and Germany would have been massive by the standards of Viking Age Scandinavia. It was also a religion associated with powerful realms such as the Roman Empire (both of them) England, Francia, and so on.

This prestige rubbed off onto the rulers who converted as they were able to tap into broader trade networks and acquire new luxury goods such as wine, which were critical in creating and retaining a large number of warriors sworn to a ruler. Leaders who did not convert such as the rulers of Lade were unable to attract as many followers to their cause as the Christian rulers.

So there were many reasons for Norsemen to convert to Christianity, ranging from the imminently practical such as greater access to trade networks to the more intangible, association with the prestigious religion of neighboring realms.