As I illustrated in: Was Harald Hardrada still a practicing pagan and only nominally a Christian?, the use of raven banner was not limited to the pagans (I cited the example that Christian monks mentioned the banner to praise the Christian (ex-)Viking King and Husband of the patron, Emma of Normandy, Cnut the Great, in the primary text). Christian also had little difficulty in the raven banner.
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On the other hand, not all the scholars agree that all of the Normans fully embraced Christianity by the turn of the millennium. It's true that the ducal family adapted Christianity and produced a bishop by the last decade of Richard I's reign (d. 996), a few scholars (though rather minority) points out that the memory of the pagan period had not totally forgotten as the fossilized past in Normandy in the first decades of the 11th century (the majority had definitely adapted Christianity by then, though, based on the status of monastic communities there).
Recent scholarship also tends to lower the degree of unification achieved by the early "Dukes" of Normandy - Rollo had perhaps in fact been just a "Count" of Rouen, with other possible power center of Scandinavian settlements in Normandy, as I mentioned before in: I'm curious about every day life in Viking Normandy around 950-1050 AD. So, it might not have been entirely impossible that "dissident" Vikings (either settlers or temporal visitors) to keep their pagan belief for the time being.
As a rather extreme example, Kaufhold argues that it took about 4 generations for the Normans in Normandy to be fully Christinanized (Kaufhold 2000). If we accept his model, the later part of the reign of Richard I and the almost whole reign of Richard II (d. 126), about two generations (about 960s-1030) would be regarded as the transition period in rather long-term process of Christianization.
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