"You should seek after things with eternal significance."
Why this belief is harmful: On the surface, this sounds like Jesus' "Lay up for yourselves treasures in heaven." But rather than meaning that you should help people and seek spiritual meaning, instead of hoarding stuff for yourself, the way it was taught to me in Mormonism is that you should never trust your own judgment about pretty much anything. It doesn't matter if something seems harmless, or healthy, or even necessary to you or others. If it's not going to be around forever, or be okay to have or do or be with in God's Celestial Kingdom, then it's worthless.
This can have horrifying consequences.
Not only does it often mean shunning friends or family members because they have "sinned" (like by drinking coffee or going to a different church), it also means believing that a large number of people simply won't exist in the next life. That all other religions, all other spiritual practices, all other forms of relationships besides temple marriages, will all be gone.
Disabled people will be "healed." Autistic people will be "cured." People who "suffer from same-sex attraction" or "gender confusion" will no longer have their "unnatural longings." Women and black men who despair because God withholds his Priesthood from them, for reasons they don't understand, will finally know and accept His plan. And the Earth will be restored to its Celestial glory, so what point is there to taking care of the environment? Better to prepare for Jesus' coming by hoarding food storage, and waiting for His judgments to be poured out upon the wicked.
This is where Internet Atheists pointlaugh, and talk about the evils of "religion." But their own neoliberal, White America centric beliefs write off a whole lot of people and things that they feel are ultimately irrelevant. Libertarian atheists basically believe that poor people all had it coming, or if they didn't that it's all the government's fault. And the most vocal Internet Atheists seem to have contempt not just for religious teachings and practices, but religious people, whom they write off as "mentally ill." Especially when those religious people are brown, and live in the Middle East.
The common thread, here, isn't a literal belief in an afterlife. It's saying, in essence, "I don't see the point of this thing, therefore it is worthless." Not "worthless to me," not "unnecessary for my life," but completely worthless in some ultimate sense.
The people who believe this tend to write off everyone that "worthless" things have value to, as not worth getting to know or understand. And they often, conversely, write things off as worthless because "worthless" people like those things.
- Twilight isn't a bad book because it teaches unhealthy, abusive relationships. It's a bad book because teenage girls like it.
- Social and economic justice issues aren't unimportant because of anything that Jesus said. They're unimportant because liberals care about them.
- The "elevator incident" isn't irrelevant for any reason that makes any sense. It's irrelevant because some brown religious people in the Middle East mutilate women's genitals.
Scratch the surface of any argument for "purity," and you'll find ageism, racism, sexism, homophobia, transphobia, theo-phobia, or just plain contempt for people who are different from you. Sometimes overt, sometimes unconscious, but always with justification which boils down to "cooties," and which tends to be painfully obvious to the people who are the targets of such discrimination.
Entire belief systems exist largely as justification for why those people don't deserve to exist, or at least shouldn't exist in the same space as us. Objectivism is this for poor people. Internet Atheism is this for religious people. Mormonism is this for anyone who doesn't outwardly conform to the Mormon culture, and therefore makes "Chapel Mormons" feel threatened. And the Free Software movement is largely this, for people who aren't technically proficient or who present as female.
The way you can tell, despite their protests, is the complete or near-complete absence of those people among them, and/or the absence of any representation or decision-making capacity for those people. Because it's a sign of how they treat them.
Forty years ago, black people could only go to the Celestial Kingdom as servants. Today, they can marry in the temple, but God now splits up gay couples. Whether your "principles" come from God's living prophet or Reddit, what's "pure" and "impure" can change, based on how many people listen to their consciences and hear out the stories of those who are hurting.
Better to get ahead of the curve, than to be suddenly left wondering why it isn't okay to be hateful anymore. It may not be an "eternal" principle, but I feel that it's better in the long run.