Here are some reasons why:
1. Its expectation is foolish.
Whether you believe America was ever a "Christian nation" or not, it is theologically naive and demonstrably false to think laws or policies make anyone a Christian. You cannot create or recapture a people for Christ by illegalizing sin. (Which, by the way, is not to say that certain sins shouldn't be illegal. It is only to say that, for instance, outlawing gay marriage or repealing Roe v. Wade won't make anybody a Christian, much less make America "a Christian nation.")
2. Its medium is moralism, not gospel.
This is similar to my point above. It makes kingdom militancy about religion, not gospel. It seeks a Christian coercion of others toward better behavior, not an incarnational sharing with others of the better Way.
3. It is theologically naive.
It is the height of weirdness to expect people who don't know Jesus to act like they do.
4. It is often hypocritical.
It is the height of weirdness to expect people who don't know Jesus to act like they do especially when we can't get our own house in order. So long as large numbers of Christians continue contributing to the divorce statistics, the porn industry, and more acceptable sins like gluttony and gossip and greed, we have zero business telling the world how to act. Judgment begins at the house of God (1 Peter 4:17). Repent, Church!
5. It battles against flesh and blood.
We're not supposed to do that. (Eph. 6:12)
6. Its treasure is temporary.
I am not overly concerned with the culture war because it is a battle for something that doesn't last. Culture is temporary. I am far more interested in the transformation of peoples through the transformation of people than I am in the subduing of culture through the modification of behavior. Nobody ever got into heaven by acting better.
7. It makes idols of comfort and safety and propriety.
The culture war is largely driven by fear. We're afraid our public schools will ruin our children, we're afraid gay people will ruin our families. We're afraid a Democrat will ruin our country, we're afraid liberals will ruin our neighborhoods. Now, there is nothing wrong with wanting to protect our family, and safety of course is not a bad thing. But neither is it a biblical virtue. Ditto comfort. Or have you not read the New Testament? I'm just gonna put this out there, but maybe it's God's design for us as people and for Christians throughout all time to endure hardship, danger, persecution, and even death. Wanting not to suffer is human. Thinking we deserve not to is unChristian.
8. It has no root in Jesus' ministry.
Jesus knew heart change didn't come through political power, cultural pressure, or zealotry, so he was keenly disinterested in those things.
9. It mangles mission.
The culture war sets the Church above and against the world, rather than in but not of the world. It turns us into picketers and politicos. It makes us suspicious and speculative and sensationalist. It takes relationship completely out of the missional equation. It turns us from peaceful ambassadors for Christ into pontificating warriors for Christianity. It does not ask us to serve and sacrifice, which are non-negotiables for Christian mission, but to maneuver and argue.
In Romans 1:5, Paul writes:
Through him and for his name's sake, we received grace and apostleship to call people from among all the Gentiles to the obedience that comes from faith.
A few things:
Paul says we "call" people. This is the work of gospel proclamation, carried out in both word and deed.
Paul does mention "obedience," but this obedience is the kind that "comes from faith." Faith comes first, then obedience. It never ever ever ever works the other way.
Lastly, and most importantly:
10. The culture war is carried out for our name's sake, not Jesus'.
I am not a fan of gay marriage or Roe v. Wade, and even though I would vote to outlaw the former and repeal the latter, neither of those actions in themselves will make a single unbeliever say "How wonderful Christ is!"
The bitter truth is that the Christian culture war is not carried out for Jesus' glory and renown, but for ours. It makes "Judeo-Christian values" the end-game, the treasure of our mission. And that is idolatry. Nobody was ever legally or argumentatively or even culturally convinced to believe in Jesus. But millions have been loved and served and submitted to into believing.
Dying for somebody says a whole lot more than debating them.
I choose the gospel. Come hell or highwater, come a liberal administration in Washington for the rest of my life or actual suffering. My treasure is not Christianity, but Christ. My hope is not a Christian nation but a Christ-saturated universe. I trust not in princes but in the King of Kings. I choose war on hell and death through the liberating power of Jesus in the glorious gospel of the grace of God.
For the glory of God.