Anyone who tries to say that the Constitution doesn't promote the separation of church and state hasn't read it. The founding fathers were extremely wary of including religion in the laws of our country having seen the troubles that came from government trying to legally enforce a religion on its people.
We're fighting against religious extremism in the form of fundamentalist Muslims, why would we want to promote the exact same sort of Christian fundamentalism in our own country? A secular government is the only way to protect the religious freedom of ALL citizens. Political leaders who try and tell people that this is a Christian nation completely miss that if the government were to promote Christianity it would put government in the religion business. For a group who says that they don't trust government this is pretty ridiculous: you don't trust government to get anything right, yet you want government in charge of promoting religion.
I've also noticed that those who want government to promote religion always assume it will be THEIR religion that will be promoted.
"History I believe furnishes no example of a priest-ridden people maintaining a free civil government. --Thomas Jefferson
"The clergy, by getting themselves established by law and ingrafted into the machine of government, have been a very formidable engine against the civil and religious rights of man." --Thomas Jefferson