Congress can do that, can't it? Why can't it? It could declare all competing religions as terrorist organisations.
Obviously it'd be immoral, unorthadox and impractical but I expect Congress has a long history of promoting and stamping out ideologies and they can do it. What little I know about the civil rights era springs to mind.
Legally, Congress cannot do that, as long as it is "respecting" some religion. It's true that the US Constitution is more used as an excuse than upheld, but at least the language is pretty plain.