That's an interesting inclusion if you're wanting to avoid appeals to authority. Why does it matter whether they were Christian?
It is a fine line between appealing to authority and pulling historical examples of lessons learned the hard way. I don't know what else to refer to those who wrote the constitution as, if "founding fathers" has some subtle whiff of appealing to authority I'm haply to refer to them as something else. The point remains, though, that freedom of speech was protected so early on based on what people of that time saw happen without free speech.