Group of atheists has filed a lawsuit

Why do you think that? Be specific.Why do you believe that "spiritual" anything can be considered "secular" under any circumstances? "Spiritual" is virtually synonymous with "religious," particularly as the term is used to describe the cross.If an object is placed in a museum because it offered spiritual comfort, then that reasoning is non-secular.If an object has a secular purpose for being placed in a museum, any spiritual feelings or reasoning is irrelevant, so why should they be mentioned?

A law that enshrines an obvious Christian symbol as a national monument would itself face the same Constitutional scrutiny as placing it in a Federally-funded museum. Congress can't actually bypass the Supreme Court without a Constitutional amendment, after all.