Paganism as a whole was largely dead by this point in Western Europe, although while almost everyone would have professed to believe in Christianity the forms this took were wildly varying, with little authority or uniformity being imposed by the papacy in Rome and several branches of Christianity in more isolated places such as Ireland would probably fit the idea of pagans I imagine you have in your head. This, however, was not the case in Normandy. Normandy had experienced a spiritual revitalisation in the first half of the eleventh century and was seen to be one of, if not the, major bastions of the papacy during the reigns of William's father and William, at least before the Conquest. William personally funded the reconstruction of several dilapidated monasteries in Normandy at his own expense and his lesser nobles collectively funded the construction of almost 20 new monasteries in the 30 years prior to the Conquest (for a frame of reference there were 9 monasteries in Normandy in 990). In addition, Normandy was a centre of the religious reforms which the Catholic Church was undergoing during this period, specifically to address the issue I underlined above about deviations in local practices from those in Rome, something which bought William a lot of leeway from Pope Alexander, who mostly allowed him (or rather didn't kick up a fuss) to personally control all significant aspects of the Church in Normandy, including most major appointments, so long as priests followed the orthodox teachings and even allegedly supported his invasion of England, although this is much disputed. As such, it is unlikely that anyone in William's army would have been anything other than Christian and fairly orthodox Catholics at that. If you want to do any reading yourself I would recommend David Bates: Normandy before 1066.