For one thing, I would encourage the Millennials to reconsider who/what the opposition is. It isn't Christianity, it isn't religion, it isn't authority.
The way I see it, metal is not so much "defiant," as it is, simply, "red-blooded."
It is the passive-aggressive, the limp-wristed, the sad-sacks, the down-and-out, the resentful, the disaffected urban intellectuals, the militant atheist liberals, those who want to make the the world even "safer," those who want to make reality even more like an amusement park, it is the hipsters and their NPR Best of Metal lists!
If Black Metal raged against Christianity, then this is what the future of metal must rage against.