Take a look at this pie chart.
https://www.prri.org/research/2020-census-of-american-religion/
More than four in ten Americans (44%) identify as white Christian, including white evangelical Protestants (14%), white mainline (non-evangelical) Protestants (16%), and white Catholics (12%), as well as small percentages who identify as Latter-day Saint (Mormon), Jehovah’s Witness, and Orthodox Christian[2].
The fascists get to pick the in and out groups. They can broadly say that the US is a Christian nation first and not solve the specifics of which christian denomination until later. They can choose white Christian as the in group and then non-Chrisitain, non-whites as the out group. Genociding and/or ethnically cleansing each group of this 56% of the population could easily take decades. But perhaps US Nazis will be more efficient and manage it in under a decade.
Assuming we are a christofascist dictatorship all that time and 56% of the population is either killed, removed or indoctrinated, the fascists will again have an obvious choice to make assuming things are roughly the same. Currently, 30% of the population is white christian protestant. That non-protestant 14% percent of the population will be killed, removed or indoctrinated.
If the US hasn't collapsed at this point or been invaded by another country after losing 70% of its population then the collapse will not be far off. At this point if only white mainline and white evangelicals protestants are left I'm sure the evangelicals will have plenty of clever ways to dissect the mainline population until they are all gone as well.
If the US is still somehow around at this point with only 14% of the population left, then the fascists will finally have to answer the question of which evangelical protestant denomination gets to be the new in group. I suspect whoever is in control of the US government at that point will bring up a colorful pie chart as I have done and plan out how to divide up this 14% of the population by denomination.
I'm going to fast forward a bit because they have to do it again once they are down to one denomination and then for a more specific denomination and then a for a specific sect and so on. Finally at long last, after somehow surviving this long, perhaps with the threat of blowing up the world with nuclear weapons, I'm not sure, the US will be down to a single Christian religion with a very specific set of beliefs.
I'm still fast forwarding slightly here. They will then proceed to kill each over eugenics. Once that is done and everyone looks basically the same, they will move on to a word I have just made up called imagenics. This is where you see who can bullshit the hardest about made up differences to reduce the population even further.
If anyone is still left in the US at this point, perhaps in an underground bunker, they will all drink the Kool-Aid and die.
What we are seeing isn't the death throes of Christianity as there will be Christians elsewhere in the world. If not stopped, this is the death throes of America. edit: typo