Look at countries with official religions: the churches languish because people figure their taxes support it. It becomes bureaucratic. Separation of church and state is good for churches.

the people languish.
This is true of European kingdoms during feudal times.
It was true of the Muslim Caliphate of circa 1,000 to 1500.
It is true of Iran.

It does not matter the religion.

N.B.
The people and the state languishing under religious domination is demonstrated in the contrasts between Protestants and Catholics.
Protestants are less dogmatic than Catholics.
Once protestants took hold in some kingdoms, they became the dominant powers - the Dutch, the Brits and then the Yankees. The Germans are more powerful economically than the French.
The pilgrims were extremely religiously dogmatic but this wore off a bit, when the Founders came about - America started to become great.

Any attempt to instutionalize any religion will eventually backfire, even as vying religions grab for control.

Best to keep them separate.

If you think about the time when the Founders wrote the Constitution and how religious people were then as compared to now, it is pretty amazing the language and purpose they used to definitively state the seperation of state and religion.
Freedom of religion for all.
Just another example of how smart the Founders were.

They are rolling in their graves at the likes of Santorum trying to ram Catholic dogma down people's throats and getting even 1 vote.

The Founders believed in freedom of choice. And that included religion.
They were not in favor of imposing a Catholic theocracy.
If you are a Catholic - that is your choice. And I have nothing against you - provided you have considered the matter and come to a thoughtful decision.

Pilgrims were very dogmatic. Founders pretty secular (for their time). Now getting more religious extremism again. Witness Santorum support. Koran burning etc.
Can anyone tell me the difference between the mad mullah's in Iran and Santorum.
Except their purported faith.
Because I can't see one.

Just to be clear I am an atheist.

to the notion :- "do unto others as u wud have them do unto u"

Most religions incorporate it in their teachings, of course, but I don't think the concept requires religious underpinnings for its moral authority. It's part of almost every ethical tradition, dating back even way before Jesus said "whatsoever ye would that men should do to you, do ye even so to them."

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That's in the Declaration of Independence. Arguably, it puts the stress on nature and naturalism; it even seems to capitalize Nature, as well as God.