The only thing that generally concerns me about Christian metal album art is how much of it harkens to Eastern Orthodox iconographry. The "darker" elements are often the most reverent, not evil or rebellious.
I can't speak for all of these bands and album covers, because I only know a select curated group of Christian metal bands.
Topics like the global Christian persecution, overcoming temptation, spiritual warefare, dealing with long ignored sin in our lives, division among brothers, and wolves in the fold; these topics deserve a musical background that has more drive, and passion than what pop-worship allows for, and is well suited to the "darker" minor tones, which also bring out a solemn, earnest, and reverent mood.
The type of Christian metal that I know is usually worshipful. Many of the examples given on this thread are actually more outreach oriented, and therefore more upbeat.
Bands like Stryper are not worship metal, but are actually evangelist metal. Like Paul, they become a Gentile to preach to the Gentiles. Like "Carman", from the late 80s to early 90s. They are culturally relevant, yet the content remains pure to the Word.
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There are too many different things going on in "metal" to be debating about it as if it is a single thing. The definition of "metal" has changed far too many times in the past 40 years.
The metal of the 70s was basically hard-style blues, and power-ballads.
Modern metal has no identity.
Many "metal" songs don't even have distortion or drums or screaming.
Stephen Curtis Chapman even uses operatic voice in his new song "Glorious Unfolding", which to me sounds like the vocal style of most Germanic/Danish metal.