A tough question to answer.
Personally, I think that "America was founded as a Christian nation," and "America was not founded as a Christian nation." are both misleading statements.
I think the founders didn't want people to be required to be Christian, as a few of them were Deists, but they didn't really want America to be quickly filled up with new ideas either. People who are hostile to Christianity resort far too much to the personal writings of Jefferson to try to lay out what our founding principles were. Jefferson was one of the few who was somewhat in the camp of not caring what people believed.
There is also the lie that most of the founders were deists. The vast majority were Christians. With any question, I think you have to work to dispel common misconceptions and misinformation campaigns.