What do people mean when they say this? Are they suggesting the US government is built on exclusively Christian principles? If so, which ones?
I would think they mean core components of Christianity like, say, the commandments, vicarious redemption, original sin, etc.
The 1st commandment is 'Thou shall have no other gods before me' which is the antithesis of American religious freedom. Vicarious redemption is the opposite of personal responsibility which is required for our legal system to function. Original sin is the opposite of a presumption of innocence built into our system of justice.
Don't get me wrong, I'm not knocking Christianity. People are entitled to their religious beliefs. It's just that Christianity as the inspiration for American government is an absurd suggestion because the two are so far apart ideologically. Christian nationalism (the US is founded on Christian principles) is quite literally un-American.
A supreme being is just like a normal being...but with sour cream and black olives.