I think Christian nationalism and politics in general is a problem. But I don’t deal in conspiracy theories. I also realize many churches are on the wrong track: Protestant, Baptist, Assembly of God, Catholic, Seventh Day Adventist and so on.
I'm not sure what conspiracy theories you are referring to. Conspiracys are by definition secret, hidden behind the closed doors of the conspirers yes? Christian Nationalism is far from secret, and has been with us to a lesser or greater extent since at least since the mid to late 19th century that I know of, being a main component of the politics of some leaders in the women's temperance movements. I wouldn't even call that a conspiracy, let alone a theory.
Today those ideas have become mainstream within very wide Christian circles, so nor could it be called a theory. It's just politics.
Now if you want to talk about a real conspiracy, the work being done behind the scenes by Satan and his agents to destroy protestantism through the various Catholic orders and societies, then we could do that. Thing is, somewhere along the line there will be found connections between the overt and the secret.
I suggest you do a little research on the current plans (overt) by the current pope, following in the footsteps of his 3 predecessors, to establish Sunday sacredness as a legislative enactment, enshrining it in law in all nations. It isn't being promoted overtly of course as a religious holy day, he knows the world will not accept that, but as a climate Sunday...a day of secular observance and rest for the common good of family, society, and the environment? Then the covert side of the issue becomes effective... Enforcing man made commandments and doctrines instead of God's. And the Christian nationalist movement will go right along with that, not even needing to care too much about the environmental side of things, but will enact legislation to support and promote Christian morality upon the nation...
Now you say that such a concept isn't in the bible. Yet Revelation 12 through to 14 reveals a number of cogent realities that coming together portrays just such a scenario if you read it without preconceived prejudices.