I highly recommend David Sehat’s The Myth of American Religious Freedom (Oxford UP, 2011).
It has long been evident that the secular liberal story told about the history of the First Amendment and how the US has historically granted individuals free religious expression is dubious. Not to put too fine a point on it, it’s mixed in with a lot of bullshit. (Maybe not as bad as Religious Right conceptions of US history, but if that’s the competition…)
Sehat’s book is the best one that I’ve encountered that confronts these myths, showing how much of US legal history exhibits a Protestant “moral establishment” that has been coercive in character. (He also persuaded me that this was much more than an “informal establishment” as has sometimes been described, and that I have referred to in the past.) In the process, Sehat illuminates much of the present culture wars and makes it much clearer what exactly the Religious Right today is aiming to recover.
I’m not going to attempt a summary; as with much historical writing, it is the details and a carefully drawn context that makes the case. But I think many US secularists would benefit from reading the book. It may even help us reduce the bullshit we produce.