Since only one signer of the Declartion was Catholic, Cahrles Carrol, and the religous backgrounds of many are questionable, your questions certainly has some interest. Many were "Christians" in a metaphorical sense, and many were diests. Many, of course had to resign from the Church of England to whom they were pledged as English subjects, until the Epsicopal church replaced it. The more pertentnint question is that America was a protestant nation at it's inceiption. We all want to believe that Jeffereson was Christian, and I believe he was. He was of course ver much influenced by the enlightenment, whose ideals were profoudnly laid out in the Social contract by Rousseau -- a Catholic. so, to the extent that the ounding of the US was a reflection of the codification of Rousseau's ideas, and that is dependnet to the extent that Jefferson was influenced by rouseau's writings, then a case can be made that the US was a christian nation. However, the fact that Jefferson can be seen as America's lawgover as defined in the social contract not withstanding, the US has been a protestant nation since it's inciption.
Carrol himself suffered and fought great prejudice, which still exists, against Catholics. At one time in maryland, Carrols colony, it was made unlawful for Catholics to hold public office. Carrol fought this and won using up much of his presitge and wealth in the process. His brother John was the first Catholic bishop in the US.
However, since only one of the founders was truly Christian, Carrol, one would be hardpressed from that point of view to argue that the US was ever a Chirstian nation. However, Carles Carrol was there, and thus so was the Holy Spirit.
The amedment against a national religion, and it's intepretation since the revolution is clearly established in law. The US is a secular nation, and all are free here to practice whatever they may call relgion. This fact at once protects our rights as CAtholics, but also allows for the practice of anything including atheiesm. Better we pray than argue.