Batter my heart, three-person'd God; for you as yet but knock, breathe, shine, and seek to mend.John Donne, Holy Sonnets
Take the question seriously for a moment, because it deserves better than the answer it usually gets. A genre whose founding gesture was the burning of churches now has thirty years of records made by people who go to one. Either that is a contradiction that cannot be held, in which case a lot of musicians have wasted their lives, or the genre is less closed than its own catechism claims. Both positions have serious arguments behind them. Neither of them is a joke.
The word itself is a surrender
Nobody in this scene calls it Christian black metal by choice. The term they accepted is unblack, and it exists because Darkthrone had printed the words unholy black metal on Transilvanian Hunger in 1994 and somebody inverted the slogan rather than the cross. That is a concession written into the name: a group of musicians agreeing in advance that they are playing a modified version of somebody else's music and will not claim the original title.

The record that put the word into circulation came from the other side of the world, and it was made by one person. Horde's Hellig Usvart, whose title is Norwegian for holy unblack, was released by Nuclear Blast in 1994, credited to a musician calling himself Anonymous, and was in fact the work of the Australian drummer Jayson Sherlock, who had played in Mortification and Paramaecium. The Norwegian title did its job: a good part of the audience assumed the record came from Scandinavia. When it emerged that it did not, Nuclear Blast's founder Markus Staiger received death threats demanding the man's name.
Norway already had one, and it did not go well

Formed at Jessheim in 1990 under the name Crush Evil and renamed in 1993 after a Latin verb meaning to call to witness, Antestor were working inside the Norwegian scene at exactly the moment that scene was becoming internationally notorious, and they were treated accordingly. The demo The Defeat of Satan appeared in 1991. Martyrium was recorded in 1994 and sat unreleased until Endtime Productions issued it in 2000. The Return of the Black Death came out through Cacophonous Records in 1998, four years after it should have. In Faust's fanzine, Euronymous was asked what would be done about the band and answered that there were plans and that they would not continue for very long. Nothing came of it. The band are still working.
We identify ourselves as black metal as a music style, not black metal as an ideology or belief.
Kjetil Molnes of Antestor, Morgenbladet, 6 June 1995
That sentence is the whole defence in one line, and it is worth noticing how modest it is. He is not claiming the tradition. He is claiming a set of techniques: the tremolo picked riff, the blast beat, the shriek, the cold production. Whether a technique can be separated from what it was built to carry is the only real question here, and it is not a question with an obvious answer.
The contradiction, stated properly
The case against is stronger than its usual presentation. Black metal was not assembled as a neutral toolkit that later acquired a theme. The people who made the second wave defined the music by its enmity, said so repeatedly in print, and acted on it in ways that produced criminal convictions. On that reading the anti-Christian content is not decoration on top of the form; it is the reason the form exists in the shape it has, and a band using the shape while reversing the content has taken a weapon and made an ornament of it.
The case for is not the feeble one usually attributed to it. It is this: black metal's symbolic vocabulary is entirely parasitic on the church. The inverted cross, the desecrated altar, the blasphemy, the Latin, the burned stave church, the priest as antagonist. Every one of those requires Christianity to be present and taken seriously or it means nothing at all. A genre that built its whole vocabulary out of a church has a weak position when it complains that the church has walked in. The heresy needs the orthodoxy. Remove one and the other stops functioning.
The heresy needs the orthodoxy. A genre that built its whole vocabulary out of a church is in a weak position when the church walks in.
The honest conclusion is that both are right about different things. The unblack bands are correct that nobody owns a picking technique. Their opponents are correct that the tradition is not only a picking technique, and that a record whose content the founders would have regarded as anathema is not simply another entry in the same catalogue. What is not defensible is treating the whole subject as a joke, which is what most of the writing on it has done for thirty years.
What the records actually do
The practical comedy at the centre of the subject is that almost none of this is audible. The vocals are shredded past the point of language, exactly as they are everywhere else in the genre, so the doctrine travels the way a smuggled letter travels: invisible until somebody opens the sleeve and reads. A listener who does not check the lyric sheet will hear a cold, fast, well made black metal record and nothing else, which is either the strongest argument that the form is neutral or the strongest argument that the content is decorative, depending on which side of the table the reader is sitting.
The music itself is more varied than the label suggests. Antestor work slowly, with doom tempos and a genuine bleakness that has nothing triumphant in it. Crimson Moonlight from Sweden built The Covenant Progress, out on 14 March 2003, around John Bunyan's The Pilgrim's Progress, which is a seventeenth century allegory set to blast beats and is exactly as strange as that sounds. Drottnar, from Fredrikstad, play a technical and rhythmically militarised version of the form and moved toward political rather than devotional lyrics on Welterwerk in 2006. Slechtvalk in the Netherlands went symphonic and folk on At the Dawn of War in 2005. Frost Like Ashes, formed at Kansas City in 2001, made Tophet in 2005 and are the most straightforwardly aggressive band in the whole grouping. The full listening order is in christian Black Metal Bands.
A parallel supply chain
What kept the scene alive was not argument but distribution. Shops would not stock the records, established labels would not sign the bands, and the answer was a set of small operations built for the purpose: Endtime Productions in Sweden, Bombworks in the United States, Psycho Acoustix, and a network of mail order that ran on the same postal economy as everything else in the underground. The bands toured the Christian festival circuit because the ordinary circuit was closed to them, which produced the odd situation of black metal being played to seated audiences in daylight.
Geography matters here too. The form spread the way australian and New Zealand Black Metal spread, by post and by isolation, with the first widely heard record made in Australia and the movement's most consequential band working in Norway with nobody speaking to them. Indonesia produced Kekal. Poland produced Elgibbor. None of these places had a scene either. All of them had a post office and a conviction.
Where it stands now
The heat has gone out of it, and not because anybody won. Omen, released by Bombworks in 2012, is a competent record by a band with thirty years behind them and it caused no incident whatever. Musicians who would have refused to be in a room with these bands in 1995 now play festivals with them. The genre has spent three decades absorbing shoegaze, orchestras, ecology and academic theory, and after all of that a Christian lyric sheet is not the outrage it once was.
The verdict, given once: the objection is intellectually serious and the records are mostly ordinary, and those two facts have very little to do with each other. Three or four unblack albums would survive on their writing alone if the lyrics were in a language nobody could read. The rest are documents rather than records. That is a worse ratio than the American scenes managed and a better one than most national scenes of the same size, and it is not the ratio anybody argues about. Readers who want the whole shape of the thing, from a Newcastle sleeve in 1982 to a farm in Washington State, will find it in the black metal story, and the American half of it is in the American scenes.