Let's think about the establishment of a real national church for a moment. From a practical standpoint, the govenrment has resources and abilities far beyond any individual Americans, or any subset of Americans. Imagine for a moment the establishment of an official national church, built upon, say, the proposition that the Founding Fathers were not merely brilliant men, but were in fact prophets, with writings and thoughts directly dictated to them by God, thus making their writings into more than simply political theory - it's considered holy. And they revise the New Testament in accordance with some sort of claimed revelation, in order to make it seem as if this was foretold in the Bible.
Now, the government can afford to do things even your national congregation probably cannot. So they do, in order to promote the national religion of the Divine Church of the Revolution - they hire hundreds of thousands of people to pass out literature, they build massive churches on every street corner, every piece of mail has a solicitation for the national church on it where a stamp would be. And not only are they bigger and better funded than your organization could possibly be, but there are some mysterious things about how the government seems to treat people. DCR members always seem to spend half as much time in line at Motor Vehicles as you do when it's time to renew their licenses, and for some reason, they never seem to get audited by the IRS. Oh, nothing you can prove, nothing you can point to and call a clear violation of the equal protection laws, but it'll be one of those things that "everybody knows", with a wink and a nudge.
And here's the fun part - you get to pay for all this, with your tax money. You, good Christian that you are, get to support this church with your tax dollars - a church that you would, no doubt, consider blasphemous. And that's in addition to supporting your own with whatever voluntary contributions you can make. Voluntary contributions being, of course, something that the national church members don't have to worry about - they gave at the office, and so did you ;)
So how long will it be before the members of your church start wandering away? Maybe it'll be the constant, 24-hour a day barrage of government sponsored proselytization, maybe they'll get tired of standing in line at the DMV, maybe they won't be able to afford their taxes and the collection plate - I'm guessing it'll be something.
That's the practical argument, against the potential results. The abstract argument is the one of individual conscience - if religion is truly a matter of individual conscience, why should you be forced to support someone else's church, one that preaches tenets that you neither believe nor practice?
Now, you could, I suppose, point out that you are already forced to support secular ideas you disagree with through compulsory taxation. But religion is supposed to be treated differently in the first place, not as basely as more worldly political ideas and theories. And the national church isn't likely to be anything quite so strange as what I described above - perhaps the only difference between it and other Christian denominations will be minor doctrinal differences, or perhaps the national church will simply be created by adopting an existing church, where it will suddenly become exceedingly accurate to say that this is a "Catholic" nation or a "Mormon" nation. But in that case, I think you can still expect to see some of the effects I describe above - bishops wielding secular power, polygamy becoming a legal lifestyle, et cetera. The whole point to the establishment clause is that two-fold protection of religious freedom that Brennan talks about - not only should you be free to practice your beliefs, but you shouldn't have to face pressure from the govenrment to believe as they believe.
I've gotta cut out for a bit, so I'll catch your other post some time later. Until then...
The first part of your post can be summed up by saying, that state establishment of a church creates a potential for abuse of individual liberties. But surely you can see that the 14th amendment only prohibits actual violations of liberties, that is, actual limitations on people's autonomy. As for your other point:
Now, you could, I suppose, point out that you are already forced to support secular ideas you disagree with through compulsory taxation. But religion is supposed to be treated differently in the first place, not as basely as more worldly political ideas and theories.
That it's "supposed to be treated differently" does not establish that not treating it differently in any way limits our autonomy as individuals, which is what the 14th amendment proscribes.