We are a Christian nation and we are not a Christian nation.
You are correct saying we are not a Christian nation, and the founders set us up as such so that Christianity would not monopolize the government and oppress people who practice other religions.
I think when people say we are a Christian nation, they mean that the vast majority of Americans are Christian. 75% of Americans self identify as Christians. It would be hard for us to be much more Christian as a nation.
75% Christian as opposed to...
2% Jewish
1% Muslim
1/2% Hindu
1/2% Buddhist
Everything else just gets smaller than that.
So when one religion is practiced by 75% of the nation and the next is practiced by 2%, I don’t blame people for saying this is a Christian nation. It is. By practice. Not by design but it is by practice.
By those numbers...yes, we ARE a Christian nation, though.