Was the US founded on Biblical principles or cultural Christianity? What do you believe?

I’m struggling looking back and seeing a time in US history where I think Biblical Christianity was the belief of the majority. I’ve struggled with this since at least college where the professors insisted it was a Christian nation and our founding fathers and presidents were Christian, but struggling more and more trying to understand this belief when as far back as I’ve studied the government seems self centered and corrupt.

- side question: when do you think USA was great?