(Paraphrasing) "Congress shall make no law respecting the establishment of religion...". Close, right?
Let's employ a little bit of common sense here. The U.S. Constitution is, from beginning to end, a heroic effort by our wise and far-seeing founding fathers to limit the power of large, centralized government. It recognizes man's natural urge toward tyranny, and tries, in every way possible, to provide us with safeguards against that tendency. Even the closing sentence of the document states that any power NOT SPECIFICALLY GRANTED to the federal government reverts AUTOMATICALLY to the states, or the people. The thread running through the entirety of the Constitution is, "The government that governs least, governs best". This means that our primary safeguard against tyranny is decentralization, and its attendant devolution of power to - ultimately - the INDIVIDUAL.
What the establishment section means, then, is that Congress makes no law in this regard because it's none of Congress' BUSINESS. Whose business is it , then? Read the closing sentence of the Constitution . It's OUR call - we the people. These things are meant to be decided at the community level. I'm not making this stuff up - read the damn thing!
So, when you have tyrannical courts overstepping their constitutional authority and telling communities which religion MAY BE ESTABLISHED (whether you call it "secularism", "humanism", or "anything but Christianity"), what else would you call it but a theocracy, or tyranny?