Christianity was a unifying force after the Saxon advent...

But when the Germanic tribes arrived they were cultural different in terms of Religion they were not christian.

The british christians may well have been divided in trying to decide the dominant christian tradition Celtic Monastic tradition, Eastern Roman or the re-emerging western / Frankish church.

Not being christian at this point gives a rallying point for native non christian factions and superficially a cultural identitiy to the disperate germanic groups. It is not uncommon for societyies to define themselves loosley on what they are not, particularly when under preasure from a more numerous culture.