I would assume that they would describe themselves as Chrisitan. If so why do they (ab)use Norse mythology for their twisted little songs?
Because they're filthy scum who want to monopolise the ability of any white person to feel good about their heritage in any way, and they probably want to tarnish politically neutral bands like my beloved Forefather. The whole bloody white supremacist movement seeks to create a state where they can emphasise negative racial differences by tarring anyone who wants to hold a positive view of race with the same brush as them, thus pulling otherwise sane people into their net. The idea is to create a state of affairs where there is no political spectrum between them and total multi-culturalism. They also want to take over Asatru, because they're mean.
Also, Vikings are fucking cool. Even nazis can see this. But they're seriously impuning. I think the ADL even lists 'Vikings' as a common lyrical theme that indicated racism. (Getting that statement modified should be a VAR policy, I think, without of course criticising the ADL, because then we'd be anti-semites. I don't like the ADL, just don't tell 'em
please.)