Why the American Constitution in not Christian

Different than is normally understood the constitutional principles in the American Constitution have nothing to do with Christianity. Here:

During a philosophical discussion with a Grass Valley City Council member, she surprised me by saying we all believe in God. She saw nothing wrong with expecting all Americans to recite the words “under God” during the Pledge of Allegiance. There was no way of convincing her otherwise.
But what if your religion doesn't believe in “God” or you're not religious at all? Why should you be expected to recite those words? Based on The Constitution, you are not.
Our First Amendment explicitly states: “Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; …”. It's noteworthy that Christianity isn't even mentioned in the Constitution. In fact, many of the founding fathers believed in deistic philosophy and Freemasonry. They were clearly not endorsing Christianity.