Scandinavian pagans in general were extremely tolerant towards other religions, because their own religion (if you can call it that) was not unified. Scandinavian paganism by its nature was pluralist, incorporating besides a shared pantheon of the more familiar gods also loads of local deities, as well as 'personal favourites'. Christian missionaries tried to play into this inherently syncretist nature of native belief by presenting the Christian objects of veneration (God and Jesus, primarily) into a native framework. Missionary schools in Britain and Francia emphasised that the goal of priests is to show the superiority of the Christian deity over the pagan ones by active 'shows' of blasphemy against the 'idols', and recasting the previously venerated dieties as the opposition to the warrior-Christ. This was conveniently close to what the ancient Roman missionaries dealt with, presenting the Roman pagan dieties not as unreal, but as lesser demonic beings. Later, after the 10th century or so, the church shifts its opinion and tries to present the pagan gods as merely human wizards, but that is a different development.

Anyway, already relatively early (by the early 9th century) you can see that Christian symbolism, particularly the symbol of the cross, is taken up in a pagan context as well. While some people have proposed that this is simply because the pagans did not understand the Christian meaning of the symbols, I rather suspect that they did know, but did not see any conflict.

However, one aspect of Christianity they did have issue with. Churches used church-bells to call to prayer (in this time not an inclusive, public act, but more similar to what Greek and Russian orthodox priests still do, a performance between the priest and the deity to which the common layman can be present, but not participating). The sound of the bells did annoy them, and the church-bell in Hedeby needed replacing at least three times, with once the bell being thrown into the harbour and once it being cut up. Similarly, the missionaries to Birka were complaining that the local population was quite willing to get baptised and participate (be present) at mass when the priests were there, but quickly reverted to their previous habits (of worshipping idols and eating horse meat) when the missionaries left again. It is only where the church had a permanent presence, around the town of Ribe, that Christianity really managed to take hold from the 800s onward.

The reverse is not so true, though. During the 9th century there were a number of decrees banning Christians from trading with pagans in the trading centers. It is impossible to know to what extent this law was actually followed by the traders themselves, but you can well imagine that if you were a Viking trader going to Francia, you better carry a little crucifix with you.

You can imagine that later, as Vikings got more involved with Britain and as missionary activity from Francia and later Germany got under way, there was in fact some conscious resistance from the pagan side. It is in this later, 10th-century context that we see a rise in the occurence of Thor's hammers in graves, particularly in Britain, and a resurgence of consciously pagan practices in burial, such as mound-building and cremation. You also see an increase in burial gifts in Scandinavia itself. However, this is a pretty late development, not so much from theological grounds, but rather in the context of local, aristocratic resistance against the rise of the Christian kings.

For more reading, have a look at 'Old Norse Religion in Long-Term Perspectives: Origins, Changes & Interactions (Vagar Till Midgard)' by Anders Andren, 2006, or a number of chapters in Brink's 2008 'The Viking world'.

As a related question, I've read that the Viking raids began, at least in part, as an attempt to weaken / punish the Christian kings in response to Charlemagne's aggressive expansion and forced conversions of Saxons and other neighbouring Germanic people. A sort of asymmetrical warfare of the middle ages as it were.

Is that theory generally accepted, or is it seen as the sort of unsupported fringe nonsense that the internet loves to latch onto?