I'll answer your question, Bambi, but you won't like it or agree with it. It says "under God," like our money and many of our original (and official) documents reference God because this was a nation founded by Christians under Christian pricipals, which was established to (among other things!) allow people the freedom of choice which God himself allows. In most cases, while trying to balance their own Christian faith with freedom of religion, they used "God" as a generic term. In the English language, "God" is both specific and generic. "Allah" or "Satan" is not generic.
Note that the Pledge does not mention Christ, and the Declaration of Independence, quoted above, talks about "Nature's God" and "their Creator."
If you choose to believe there is no God, what does it matter? If I did not believe in Satan, it wouldn't matter to me if the Pledge said "under Satan." Since I do happen to believe in both God and Satan, I would object to it.
There was no "separation of Church and State" until that doctrine was established by the Supreme Court.