Looking at history from the point of view of Britain, the two big periods of post-Roman invasion - the Anglo-Saxons settling and driving the Brythons to the fringes of the isle, and then the Danes conquering most of the Anglo-Saxons but failing to achieve the same level of settlement/acculturation - are somewhat comparable in theme. Both are always presented in histories as pagans vs. (Catholic) Christians. Indeed traditional English histories usually seem to identify and sympathise with the Brythons because they're Christian, even though the Anglo-Saxons (who, of course, converted in their turn) are the English, or where the idea of England comes from.
I thought it would be interesting to change the character of one or both of these invasions. In OTL around this time you had the Franks, Visigoths etc mostly becoming Arian, and the conflicts and invasions in western Europe were mostly framed as Arians vs Catholics rather than pagans vs Christians. So what if the invasions of Britain were like this as well?
I think the Danish one is more interesting, because that in particular is always framed in religious terms. What if some Arian missionary (or several) had managed to convert Scandinavia before the Danish invasions began?