Christianity is not the only faith nor the only group of people who believe in G-d -- this is especially true considering the booklet itself comes from the National Council of Churches. They've generally been a very inclusive and pro-interfaith organization, and they certainly don't tend to be folks who hold the view that Christians are the only valid faith or the only true believers.
The booklet itself is great, and I'd recommend checking it out. The NCC folks who launched the campaign seem to have been really clear that it's geared toward Christians and is rooted in Christian theology. Apparently though, the Care2 people think "Christian" is a dirty word or something. I'm wondering if maybe they thought "people of faith" was more inclusive than "Christian," but the way they use it, it's actually just offensive, since you then click through to a book that's clearly Christian. Why not just say that it's a Christian campaign?