There are only so many times you can have a White Russian before it gets boring, or indeed you stop abiding.
Islam cause I'm from a Muslim family, and it's the only religion I know anything about
Whole internet out there and most of them put broad synopsis of things out there for all to see if you want.
Old Norse Gods. Pretty much the only religion that inspires heavy metal music instead of demonizing it. Call me a sinner all you like, I'm sinning to a bitchin' soundtrack and frankly that's all that matters. Also the Valkyries, a legion of hot badass women warriors. And Valhalla, one massive party with meat and beer.
Seen some pretty decent ancient Japanese gods takes on heavy metal (and if the stories themselves are not ripe for it then we must have been reading different things), sadly I don't have many names here (mostly watched them where they had Japanese names. Can start with Metalucifer if you want though, even if they are more general heavy metal). Personally I like the Japanese folk done with electrical instruments thing more but play it how you will.
Roman has some good stuff
in addition to all the fun Latin stuff, though I suppose that is more medieval for most of them. Still good though
Old norse is also not all of paganism and old folk which provides a decent chunk of things there -- Finland, Estonia*, Germany (debatable as to where this falls under Norse vs not -- most would have that Wodan was Odin by any other name. Though there are still options** ), various flavours of ancient Britain (loads here but can start with the originators in Skyclad if you want), Russian paganism is a fun one***, there are some truly ancient ones that are not Norse at all, very eastern Europe has some increasingly doing things. Seen a few French, few Dutch, few Hungarian, less Spanish but some are trying. Plus whatever it is that Brazil does.
*
The cartoon in the background there being what is arguably the Estonian national epic.
**Probably not quite metal compared to some of their other stuff but for a starter
***
Anyway this is not folk metal/pagan metal music appreciation thread. Just mostly here to say there is way more to things than just vikings, though they do have some good stuff.