Black metal can exist with Christian themes; this is absolutely UNARGUABLE.
Here...
Let us say you heard a band you knew nothing about. They are obviously playing black metal. They have the sound and the production, the atmosphere, and definitely what sounds like a vocalist spewing out Satanic hymns of left-handed hatred or whatever the fuck you are on about. Catch my drift? What is your first impulse? TO CALL THEM BLACK METAL. You can make this judgment with absolutely any style of music, provided you know enough about its history, evolution, and progression (you are obviously quite stuck in the past on these last two, and probably on the first one, too, but I can't necessarily judge that here, and you seem to have a decent grasp on what happened UP TO A CERTAIN POINT). ANYHOW, Mr. Absolutism; you eventually discover that this band has members posing for their band picture in front of a crucifix. Still black metal, right? I mean, c'mon dude! There is totally a cross there. They are celebrating Jesus' FVKKING VNHOLY DETH. Right? Then the logo has some "Satanic" innuendo which is really just obscure Christian symbology (which you would PROBABLY be absolutely hilariously inept at noticing, by the way!). Their lyrics are even about killing some lord! Ambiguous, ain't it? Their name is clearly "Satanikiller"; this much you can make out from their logo. What's a poor boy like you to do? EH, MR. ABSOLUT BLACK METAL FAN WITH PERFECT KNOWLEDGE OV 666?
Prove me wrong, seriously. Try it!