But this blog entry I saw at Beliefnet really nailed the motivations of people wanting to declare the US a Christian Nation.  

Here is an exerpt, but check out the original post (its not lengthy).

Abner1 says, "...Well, our nation is by the people, of the people, for the people. IMO what is going on here is that people are trying to qualify this: an officially Christian nation is by the Christian people, of the Christian people, for the Christian people. Or, in other words, it is not by non-Christian people, it is not of non-Christian people, it is not for non-Christian people."