There are three ways to change the United States Constitution:
1. 2/3 votes in both Houses of Congress on a proposed amendment with identical language, followed by ratification by the Legislatures of 3/4 of the States before a certain expiration date (usually 7 years). All but once, this is how the Constitution has been altered.
2. Go state by state and get 3/4 of the Legislatures to ratify an amendment. In this approach Congress and the Federal government are excluded from the process altogether. This has been used once, to pass the single worst amendment of all, Prohibition -- the ban of alcohol, which was championed by a bunch of religious fanatics. After a couple violent decades the Feds and States finally repealed this one.
3. A vote by 2/3 of the Congress to convene a Constitutional Convention, which would include representatives of all states, who would then alter the Constitution. This is a radical option, and must be avoided no matter what, because once that Convention is convened, there are no restrictions on what they could do, so long as they can muster 3/4 support of the 50 delegations.
Most people don't realize that the current Constitution came about under a set of circumstances like option 3... the Congress of the American States under the Articles of Confederation (the first government, which ruled America from the end of the Revolution to 1789) called a Convention to discuss altering the Articles to allow for a stronger presidency. Instead, once the Framers got to the Convention, they closed all the doors, issued gag orders on everyone there, and threw out the Articles, and drafted a wholly new Constitution. This grossly exceeded their mandate, and was actually likely illegal, but the situation under the Articles was untenable.
The irony is that today we would consider this action a "palace coup," and the US might not recognize the "new" government that would arise from such illegal acts by a convention. Yet here we are, over 200 years later, living under that "illegal" Constitution.
As for things the ACLU has done for Christians... just last week there was a case involving Christian firefighters who protested being forced to march in a gay rights parade. They called the ACLU, and it got involved... too late to help them, but it did bargain a deal where from here on out the city will not force firefighters with religious objections to march in such parades. It was a town in the Northeast, I believe. I'm too tired to go look it up at this point.