The raids on the Frankish kingdoms and the British Isles brought increased contact with Christianity. Although Vikings often seem to have maintained their beliefs throughout the periods of their raiding, there was considerable pressure to convert to Christianity if they wished to have more peaceful relations with the Christians. This could happen on a political level, as in the Treaty of Wedmore in 878. The treaty bound the Viking leader Guthrum to accept Christianity, with Alfred of Wessex as his godfather, and Alfred in turn recognised Guthrum as the ruler of East Anglia.

Another more or less formal convention applied to trade, since Christians were not really supposed to trade with pagans. Although a full conversion does not seem to have been demanded of all Scandinavian traders, the custom of ‘primsigning’ (first-signing) was introduced. This was a halfway step, falling short of baptism, but indicating some willingness to accept Christianity, and this was often deemed to be enough to allow trading.”

http://www.bbc.co.uk/history/ancient/vikings/religion_01.shtml

This was similar to the Vikings/Rus who sailed to Constantinople in the 900s, creating havoc as they went and accepting great tribute. The choice of religious conversion was pragmatic for the tribal leaders.