In some ways, it seems, the Viking Age really stimulated the development of Norse paganisms. Opportunity, not religion, seems to have been the early motivation for raiding monasteries. But during the later 800s, and particular in conflicts between the Great Heathen Army and the kingdom of Wessex under Alfred the Great, religion took on a prominent role.

The occupation of Repton over the winter of 874/5 is an excellent example. The viking war band occupied the church and integrated it into their defensive perimeter—it was, after all, a stout stone structure. It was probably also a rich place for plunder, since the Mercian royal family had been intimately connected to Repton for generations. The Anglo-Saxon Chronicle is pretty laconic about this event, but Asser's Life of Alfred connects it explicitly to the expulsion of the Mercian king, who departed in exile to Rome. Anglo-Saxon kings (and their Frankish counterparts as well) were intimately connected to the church, and the example of Repton shows how attacking a royally-sponsored church could be a key moment in the conquest of a kingdom.

But even though the men who occupied Repton didn't respected Anglo-Christian ideas of sanctity, they nevertheless don't seem to have had a shared consistent ideas about their own religious practices. They established two separate cemeteries for the men who died that winter, with the men in one cemetery buried beneath burial mounds and the men in the other cemetery cremated before burial. Either kind of burial can be labor intensive—building a mound vs. building a fire—and both created lasting memories, either through the enduring monument of a burial mound or through the memorable moment of a funeral pyre.

To me, this indicates that as viking activity took on a larger scale during the late 800s, people with diverse non-Christian practices began to interact more intimately. This hadn't been a problem during the early Viking Age, when war bands were smaller and likely came from a single fjord or region. In the case of Repton, the army appears to have been diverse and subsequently fractured, perhaps reflecting the same divisions that separated the two cemeteries. But elsewhere, these multi-ethnic groups of Scandinavian emigres formed new communities that coalesced.

In some areas, Norse immigrants integrated with Christian communities, which seems to be the case especially for northern England. The Church was, after all, a dominant landholder, and so aspiring Norse-speaking kings brokered for power with the Church, just as their Anglo-Saxon predecessors had done before them. Pagan-style burials are almost absent in northern England, although there are a few graves with Scandinavian artifacts that appear in old Christian cemeteries. For the UK, this seems to indicate large-scale accommodation with Christianity.

In other places, for example on the Isle of Man and in Ireland, Scandinavian immigrants found the opportunity form new creole cultures that were distinctly pagan.

This would not have been a moment for pagan missionaries—since the whole concept of 'mission' depends on the notion of an institutional church—but locals who sensed which way the wind was blowing might very well have turned to the religious practices of their immigrant neighbors. There is at least some reason to believe this was the case for Ireland, since the Norse settlements there tended to integrate fairly well with their hinterlands, suggesting that they reached a sort of cultural balance with the peoples living around their longphorts that developed into towns.

Furthermore, it's worth considering that Viking-Age Scandinavians adopted key ideas about what 'religion' should be from the Christians they interacted with in the west. That is to say, contact with Christians may have encouraged vikings to elaborate or establish ideas about creation, the last days, and the afterlife—which are all key aspects of both medieval Christianity and Norse mythology as it comes down to us today. Furthermore, images of Christ suffering on the Cross, which vikings would have seen in churches stuffed with books, may have stimulated interest in stories about Odin sacrificing his eye and hanging on a tree to learn about runes.

That's not to say that Norse mythology was 'made up' in response to interactions with Christians (for example, I believe one-eyed figurines begin to appear in Denmark during the 600s), but it is worth considering how Christian influence may have guided Norse-speakers to cull or elaborate certain mythological stories. We tend to lack contemporary evidence for the religious beliefs of the Viking Age, but one important marker that developed around 900 was the Thor's hammer pendant. This seems to have been a direct response to the Christian practice of wearing cross and crucifix pendants, and it spread across all Scandinavia.

So I don't think there was any Norse-pagan equivalent to Christian missionaries, but it seems apparent that there was at least some conversion in both directions. And just as importantly, a lot of what we think of as Norse paganism developed during this period, which lets us think about how viking activity and the development of the Viking Age more generally may have caused Scandinavian paganism to grow in the directions that it did.

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As a final note, insofar as I know, the saying "On land we pray to God, at sea we pray to Thor" does not originate from the Viking Age. It does, however, capture the pragmatic approach to religion which seems to characterize much of the early middle ages, and not necessarily the Viking Age in particular.

Im now interested to know why Thor was associated with the sea