I believe that rulers should obey God, and respond appropriately, within their context and era, to the fact that God has revealed himself in the Bible, because I believe the same about everyone else.
This, though, seems to me to be far from what brethren who promote "Christian Nationalism" are really talking about.
In my evaluation of what they're really doing, such brethren are often simply engaging in empty displays of beating their chests in order to persuade themselves and/or others that they are manly. i.e. They hold the doctrine in order to position themselves tribally in their largely online in-house debates with other Christians. It's not because they feel called into serving God in government, and are working on a realistic plan for that. They're largely trying to build their tribe, and this is part of their positioning. If you are genuinely trying to understand how to serve God in government today as a faithful Daniel or Joseph, then theories of building Christian nations whose constitutions begin with a recitation of the doctrine of the Trinity, detailed right down to how many times you will lash blasphemers for their third offence, do not constitute practical help for anything you're likely to be getting up to, and such help was already available in the resources of the mainstream Reformed faith.
But let's leave questions of motivation, feasibility and what would actually be a plan to accomplish this in practice aside. (I think if you really believed in "Christian Nationalism" then your main priority ought to be white-hot gospel-preaching to reach the unconverted for the next couple of decades instead of wasting time upon so much online chest-thumping). Let's consider the ideas of "Christian nationalists" in practical experience.
Here, Bob Smietana explains what these types of ideas have usually/historically meant in practice. Here too is a book centred around Roger Williams and the New England Puritans which I read recently. These are good resources. In practice, attempts to build Christian nations have meant Christians being hypocritically persecuted, and non-believers (such as the Narragansett Indians and others) having the gospel presented to them being mixed with all sorts of ungodly and carnal power-plays. That's not a one-off. That's what happens. When believers at the Reformation concluded from Scripture that infant baptism was unbiblical, Christian-nationalist-minded Reformers called for their executions, and drowned them.
We're told that that's Christendom 1.0, and that what we should now be aiming for is Christendom 2.0 in which this should play out differently.
Please pardon me for laughing at this. Very funny.
We can all sit behind our keyboards and dream out our utopia, and explain why "it'll be different this time" - because people so wise as our good selves have now arrived in the world, and we'll implement so many wonderful safeguards and checks in our visionary kingdom. I don't believe a word of it. We're still fallen, and if "by their fruits you shall know them", we may judge that the church today doesn't yet show anything like the maturity to be asking God for dominion in any other realm, so let's not confuse our adolescent day-dreams with our calling to apply Christ to all of life. If there's a time for the church to turn large amounts of energy to understanding and discussing what to do when people are asking for a society run on Christian principles, then that time isn't now - we are at a different stage of development entirely. And our grown-up duty is to live in the age in which God has placed us, not a mythical one of our fevered imaginations.
It has to be said that whenever our Presbyterian brethren start discussing what kind of Christian utopia they're going to govern when their dreams transmute into reality, we Baptists do not detect the beautiful fragrance of the humble spirit of Christ, embracing suffering and lowly service before glory, but the unpleasant odour of an unsanctified lust for worldly power. (And yes, such an unsanctified lust runs free among Baptists today too; the phenomena of independent Popes ruling their local church domains is well known). It reeks of wanting to sit at Jesus' right hand before picking up the cross. The world in 2024 is one in which we're called to pick up the cross, and the mature will surely discern that instead of discussing their schemes for governing the ungodly in the civil realm.
"But, but, this just means you don't believe in King Jesus".
I believe in King Jesus. I just don't believe that I've met the Presbyterian or other brethren who are his appointed representatives on earth, destined to rule over the civil realm for us, and we will be thankful for it. Inevitably, Christian nationalists in history move very quickly towards identifying the rule of King Jesus with their own laws, and as night follows day, those who oppose the "rule of King Jesus" must be punished for their blasphemies. Again, as Baptist, we've seen how this plays out in practice, regardless of what you say in theory. No thank you!