Wrecking the churches and diminishing Christianity is part of the plan
You're looking at this through a purely religious orientation lens.
This isn't about that, this is about three things: US Constitutional Law, the Leftist Ideal of Submission of Church to State (which they CALL Separation of Church and State in order to confuse the matter), and preventing moderate and politically unaware Christians from waking up to how bad things are.
The first part was touched on, the last bastion that prevents the Left from outright criminalizing traditional moral opinions in the US is the First Amendment. In Canada and Europe they have been waging a fairly successful war against traditional moral opinions via Hate Speech Laws. This effort was also attempted in the US (and actually originated here I think) and was originally part of the US Civil Right movement, meant to criminalize speech by organizations like the KKK. However, these laws were to broadly written and in the 90s "Hate Speech" as a legal concept was pretty decisively killed by the US Supreme Court under the 1st Amendment's Free Speech clause*. This has been a bugbear even as the 90s also saw one of the most damaging events to Freedom of Religion in the US that set the groundwork for their next wave of attacks.
Being unable to outright ban speech, the left instead turned to a more subversive method: attempting to control speech via Civil Rights Laws. They have attempted to make it so that speaking against their favored groups (be it racial or sexual) creates a "hostile environment" that violates their civil rights (this is also why you saw the creation of the idea of "microaggressions"). The problem is that US Civil Rights laws have long held carveouts for Religious beliefs. IE, a Catholic School could of course discriminate by not hiring non-Catholics, Churches of course could say anything they wanted based on their scripture, etc. Further, because their most recent weapon de jur against civilization has been sexual deviants, which most religions have explicit teachings condemning, even private companies compelled under Civil Rights law would have trouble enforcing their Speech Codes because, well, a Christian who says "Same sex sexual activity is a sin and I don't think people should do it" is expressing an truly held religious belief and thus a company that fired them for saying that could actually held liable for discrimination based on religious belief...
This has led to the massive push for Progressive Christianity and its artificial amplification in media and in elite discourse specifically to discredit traditional Christianity and it's morality, it's to discredit the idea that such speech is religious in nature and thus remove it's protection under Civil Rights laws, and instead make it purely into speech that is "harmful" to a protected minority group... and thus able to be punished under those same Civil Rights laws.
The second aspect of this is the Leftist's inherently misrepresentation of the idea of Separation of Church and State, in which morals and ideals that come from religion are inherently considered suspect and illegitimate to base laws off of. Under the Leftist's construction Religion is meant to be entirely private and only apply to a person's personal spirituality, it should not effect their day to day activities (this is why they perniciously express the religious protections in the US First Amendment as "Separation of Church and State" and "Freedom of Worship", rather than the more accurate "Anti-State Religious Establishment" and "Religious Free Exercise", as "Religious Free Exercise" makes it clear that the Right in question is much more broad than just worship). Again, by having a kept religion that calls itself the same as original but teaches completely opposite things they can then claim that the original's teachings are not coming from the religion (and thus are protected) but from something else that's not protected.
And finally, it's meant specifically to keep the people in the middle asleep and from associating with the "bad" side. I see this frequently among those who don't actually understand what's going on... people reposting claims that those who support XYZ politically right positions are "not real Christians" or that "real Christians" support ABC Left wing positions. This is meant to message to those who identify as "Christian" but who are not politically and religious astute that being a good Christian is in line with what Progressives want, and that people who don't aren't "real" Christians.
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* I REALLY NEED people to recognize this. So many Millennials mistakenly believe that many left wing organizations were all for free speech until recently, but no, they NEVER have been, they've ALWAYS been pushing legal speech bans they openly talked about and made laws about this as far back as the 1970s and it was only as a side effect of the Republican Long March through the Supreme Court that we secured Free Speech from being eroded by Hate Speech back in the 1990s and 00s.
Oh I'm well aware of that. My broader point was that
these efforts aren't working. The Left(-Hand-Path) has historically been great at deconstructing and destroying, but not at creating effective and popular replacements for that which they've torn down, and this creative sterility most certainly extends into the religious sphere. In fact, I daresay that their efforts to create a subservient church or churches to function as their religious arm and decisively spiritually neuter Christian opposition has been by far the culture war front where they've had the least success. Every church that has bowed to the modern managerial state and the culture it's manufactured is well in the process of withering to nothing. The only churches having success at retaining & growing their membership are those who firmly set themselves apart from the Left's corrupting tendrils or, increasingly, even in active opposition to them.
I was reminded earlier that Rob Reiner most recently produced a 'documentary' called
God & Country specifically attacking the Christian nationalist strawman and trying to tie American religious conservatism into this newly-defined boogeyman. They got all sorts of purported 'good Christians' in there to pontificate against the mean, obscurantist, reactionary dreaded Christian nationalists and saying
the latter are the ones perverting God's church, that in fact doing the Left's bidding and marching in lockstep with the prog agenda is the most Christian thing Christians can do; IOW exactly as you say, that the latter are 'not real Christians', basically.
Well this documentary was released two weeks ago, and...it was a total dud. $38k opening weekend, not much better in the days since despite a hard marketing push and rave reviews (92% on RT) from the ever-trustworthy professional critics, and I'd bet $15 that the vast majority if not the entirety of the audience who paid tickets to see this thing were all already diehard liberals who agree with everything Reiner & his handpicked puppets have to say, and who probably aren't even Christians just like Reiner himself (or if they are, they belong to the same progressive churches that he drew his talking heads from); meaning it's a complete waste of time & money that isn't persuading anyone in the center (much less converting 'the enemy') of either how much of a threat Christian nationalism is or how Christian conservatives somehow aren't 'true Christians', as opposed to the supremely enlightened and compassionate types Meathead here brought on board. It's practically the
Bros of anti-Christian documentaries. The masses simply aren't buying what they're selling.
As I'm sure you know, the modernist churches have been crumbling while the fundamentalists, traditionalists, whatever you want to call the ones aligned with the Right by default (since the Left is axiomatically hostile to any attempt at being a genuine Christian and has been since its inception in the French Revolution) have been growing, for a good while now. I remember jokes being cracked both online and IRL about the proto-woke direction the UCC was going in up here and the likes of the Episcopalians doing the same in the States, how they'd think nothing of draping the Pride flag over the cross and having drag queens read the Bible to kids...in like, 2010. Now even as they praise the Great Queer One from their altars, their pews are emptier still than back then (they were already quite empty before) and their names are worse than mud, attracting not merely mockery but downright contempt from every practicing Christian I know regardless of denomination. And this trend shows no sign of reversing in favor of the Left & those hollow churches that have agreed to become its pets.
(Attempts by the Left to construct its own subservient church directly rather than subvert the mainliners have pretty much all failed, as well, going back to the French Cult of Reason/Supreme Being. Jim Jones tried to create commie-Christianity, got his hooks in deep with the Californian Democratic establishment including figures as high as Dianne Feinstein & Harvey Milk, then inevitably spiraled as sociopathic cult leaders tend to until he went full Jonestown and suddenly Feinstein et al. were busily pretending they never knew him. Inasmuch as modern intersectional leftism can be called a cult, it's failed to produce sympathetic martyrs or truly inspirational figures - only nihilistic murderers like Audrey Hale who they then have to bury and claim no affiliation with because of how horrible they'd look to even the most spaced-out normie, failures like Aaron Bushnell whose attempted self-sacrifice didn't further their purported cause and got them shat on by their own 'allies' anyway for belonging to Black Categories, or are exposed as not martyrs at all too quickly to stick in this Information Age like Dagny Benedict. Past, more successful martyrs like Matthew Shepard & Marsha P. Johnson are having their myths demonstrate a lack of lasting resilience as even left-wing outlets like The Guardian have had to admit their story have holes big enough to drive a 747 through & intersectional fights to claim their memory are breaking out more recently, respectively. And that's all just in America - abroad, even the cult of Lenin/Stalin couldn't carry the Soviet Union for a full century.)
One of the more interesting socio-political aspects to American Christianity I've found is the cycle of Great Awakenings. There's always this massive wave of Christian revival across the country after periods of spiritual drought & decay, most recently from the '80s to the Bush Jr. years in response to the rising secularism, hedonism and bloody turmoil of the '60s & '70s. The infamous Evangelicals who drove that awakening didn't actually have the influence they were painted as having by a media hellbent on making them out as these giga-cringe backward barbarians (I daresay the likes of Hillary Clinton, Tipper Gore and Joe Lieberman did far more to not only bash but to actually try to ban or otherwise suppress violent videogames and 'controversial' music). And in hindsight, they really were not all that militant for a religious movement, not even compared to previous Great Awakenings (like the one which drove abolitionism in antebellum America, spawned John Brown and contributed a bunch of famous/notorious Union officers during the ACW). But perhaps they didn't have to be, since the modern American Left's assault on Christianity wasn't yet in full swing then either - sure SCOTUS had banned school prayer and handed down Roe v. Wade, but you didn't have pregnancy centers being firebombed on the regular and pro-life activists getting arrested by SWAT/FBI teams in front of their family back then AFAIK.
The Left has to date consistently failed to build up a subservient church or churches that's at all popular and is any good at helping them suppress authentic Christian voices of resistance from within the tent, they can at most damage & diminish any church dumb enough to get in bed with them. They are failing still today, their latest power-play (this one aimed at the Methodists) creating a schism and leaving them with a rump UMC doomed to wither and die like the Episcopalians or Lutherans rather than getting them the whole thing like they hoped, and by all indications will continue to fail in the future. If anything even the mildest and most culture-war-averse of normie Christians are starting to get sick and tired of Leftism's tendrils snaking in everywhere - a popular (certainly vastly more popular than Reiner's poor excuse of a documentary) Christian TV series called
The Chosen came under enough fire from its own audience for having rainbow flags on set for last year's Pride Month that the creator, Dallas Jenkins (himself the son of a prominent Evangelical writer, one of the co-authors of the infamous
Left Behind books no less!) had to make a video to assuage their concerns afterward. I guess we'll find out this June whether that lesson stuck.
And I must say, I am really hoping I live long enough to witness the next Great Awakening, as much out of curiosity as out of faith. (Yes, I'm also aware that the Left will try to redirect any such organic wave of energy for its own purposes, towards its pet churches and/or intersectional deconstructionist ideology. But as I just spent 2k words rambling about, I doubt they'll be very successful. The cause and its martyrs have lost and are continuing to lose a lot of luster.) If it's politically aligned with whatever succeeds Trumpism (ala the Moral Majority with Reagan, but ideally more successful of course), as I suspect it will have to be for the sake of simple survival against a militant and fully mask-off Left...perhaps the leftists of today are more right to fear 'Christian nationalism' than they know, even if the ideology that ends up actually mobilizing under that name (if it so chooses) bears little resemblance to their strawman.