Roger Ebert,
http://www.suntimes.com/output/ebert1/cst-ftr-passion24.html
THE PASSION OF THE CHRIST / **** (R)
I said the film is the most violent I have ever seen. It will probably be the most violent you have ever seen. This is not a criticism but an observation; the film is unsuitable for younger viewers, but works powerfully for those who can endure it. The MPAA's R rating is definitive proof that the organization either will never give the NC-17 rating for violence alone, or was intimidated by the subject matter. If it had been anyone other than Jesus up on that cross, I have a feeling that NC-17 would have been automatic.
http://www.suntimes.com/output/ebert1/cst-ftr-passion24.html