Have you seen the movie? I haven't. That being the case i doubt that i am qualified to comment specifically on the content of this movie and what kind of rating it deserves based on that content.
I do think the mpaa rating system has generally been at least a bit full of shit. Boobies will get you an R, a coulple of f-bombs and a murder or two won't.
Here's info on the ratings board:
http://www.filmratings.com/about/content.htm
Even though it is in a highly self congratulatory form, there is somewhat useful information. The board is made up of 8-13 people. I wouldn't chalk a decision made by 8-13 people in los angeles up to a "war on christianity".
Personally, depending on the particular brand of christianity being represented, i'd rather my daughter see some boobies or some violence than some ultra repressive, self hating christian arrogance*. I'd be more comfortable with my kid seeing saving private ryan than any movie espousing the belief system of pat robertson.
*I'm not saying all, or even most of christianity conforms to this criteria.