Acts20.com

..sorry....but, I have noticed that Catholicism was pretty much the only religion vs. Protestantism.

Because that period pre-dated the Reformation by about 700 years. If you were a Christian then, you were Catholic. The Vikings themselves were converted to Christianity largely by the efforts of the Celtic Catholic Church of Ireland who won them over from conquerors to conquered from the early to the middle ages. In fact by the time the Normans reached Ireland in the late 11th century, the large Viking population of Dublin, Galway, and other cities had already been Christian for centuries. While all churches of the time had similar forms of worship, especially around the Eucharist, they didn't really coalesce into the Roman Catholic church as we know it until really around 1000 or so when the Bishop of Rome began to claim primacy over all others (despite the claims of modern Roman Catholics). But that's about the best summary I can give as to why these films would depict Christians as Catholic because that is essentially true, albeit not necessarily Roman Catholic.