Why do Christians keep insisting this country was founded as a Christian nation?

I been reading up on the subject and most of the first Presidents held views that Christians would consider unorthodox. Many of them were deists and most did not believe that Jesus was the incarnation of God. Yes, there were several that went to church regularly, but their beliefs were not orthodox in the slightest. Actually, while there was Evangelical Christianity at that time, not one founding father was an Evangelical. So, why do so many Christians get this idea in there head that the founding fathers were making this country a "Christian nation"?