I read the description of it on Wikipedia. I read the commentary. I saw the film itself.
While it is morally repugnant, it is that way so it can convey many messages which I believe the Church should agree with.
As I understood it, "The Life of Brian" insisted that it wasn't Jesus who was the idiot, nor Brian.
It was the people.
It portrayed the perceived stupidity of many modern Christians who follow their faith without finding out what Christ actually taught about morality and faith (reading the Bible). It reminds me of a conversation between a priest and his newly initiated curate which went something like this:
Priest: So do you like me?
Curate: Yes, Father, a lot.
Priest: Then do you know me?
Curate: No, Father.
Priest: Heh. He likes me a lot, but he doesn't know me at all.
LoB also lampooned denominationalism, which is rather silly when you look at it. I was reminded in it that Christians of all denominations have far more in common than they do have differences.
And I do think the references to crucifying were a misguided misunderstanding of the importance of the cross. It's not the cross we worship; it's the man who died on that cross.
All in all, though, I do believe there were many, many good points in the movie, such as the distortion of religion to favour politics (as exemplified by Judith), the way people get hung up on trivial things (such as when they fight over his sandals and his gourd), not to mention some politically motivated satire and the aforementioned points.
So, why, aside from seeing some brief but rather unnecessary nudity, is it rated "O"?