This thread reminds me of the hilarious Simpsons episode where Bart substitutes Iron Maiden’s ‘Garden of Eden’ for the organ music at their church.
I loved that episode and how the little old lady organist exhausts herself after playing!!!
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I do think there's a point where music becomes about the performer and entertainment rather than about glorifying God. I would say that threshold is much lower with heavy metal music than with other styles, but I don't think musical style should be the only consideration.
I’ve heard classical Masses that I don’t think should be played in church - Verdi’s Requiem, for example, seems to me very clearly to belong to the concert hall and not in a church. And the Church agrees - there are some classicial composers whose masses have in the past been stated to be unsuitable for liturgical use.
This is very truly. I don’t know how often his Requiem was ever played in church, but it was at least twice – first ever played on the anniversary death of an Italian writer, Manzoni for whom Verdi composed the piece, and I believe it was played for Verdi’s funeral. For this mass and other masses there is no way that a trained singer can’t sing it without going full-blast operatic. I know because I’ve done a couple of pieces from the mass. I’m sure Verdi put all of his heart and soul into the piece and that it was spiritual for him, but was appropriate for mass? As an musician who sings opera, lieder and sacred music, I would say no. You can’t escape the “secular” style because the music itself “demands” you to sing it like you are singing an opera. There are other masses composed throughout the centuries where the music does not demand that kind of singing. You still need trained voices, but there is no need to sing it like you are on the stage of the Met.
Also remember that many masses composed in the 19th century were not meant to be performed for mass, but as concert pieces. So, although they are religious and can be very spiritually uplifting, that doesn’t mean they are appropriate for mass. I love and have been spiritually uplifted by a lot of religious/sacred music which may or may not be appropriate for a mass.
But in regards to heavy metal music and basically all genres of music, I think one needs to see if the way you perform the piece or if the style settles too heavily on the secular end. Did the style first begin in the secular genre? Can it be performed in a way that will not evoke a secular performing style? My colleagues and I have named some hymns which are standard in churches today as “broadway” hymns. Yes, they may pull out spiritual emotion, just like many classical/operatic religious works not appropriate for mass are pulled from me, but they sound like they could be from a broadway show. As a musician we try our best to not sing it like we’d be on broadway, but the compositional style, itself, is done so that you really can’t tone it down, so to speak.
I think one of the reasons why chant is held so high is because even before Christianity, it was used in religious worship in all parts of the world. Eventually, I believe the secular texts were used for chant (I have to double check my cd collection, I recall owning a cd of chant that was all secular), but it was first and foremost used for worship. Chant isn’t even really “classical” music. The classical genre and the 12-tone scale were eventually derived from that such as polyphony, etc. and in the beginning most classical music was sacred music, then it proceeded to incorporate secular music. I certainly do not want only chant for mass. We have so many centuries to today of appropriate sacred music and I’m happy for that.
My personal opinion on heavy metal used for mass…. I believe that since it has its roots in the secular realm of performance and that the style in which you much sing it and perform it has always been secular, it falls too heavily on that end.
Really, the danceability vs. just-for-listening argument goes a long way back before Iron Maiden. Try dancing to Yes’ “Close to the Edge”. 
