A: Morally unobjectionableAt some point, the B and C ratings were combined into a new O rating for "morally offensive" films.
A-I: Suitable for all audiencesB: Morally objectionable in part
A-II: Suitable for adults and adolescents
A-III: Suitable for adults only
A-IV: For adults with reservations
C: Condemned by the Legion of Decency
The difference between a great but disturbing film "suitable for adults" (A-III or A-IV) and a film that got an "O" for Offensive, was that the "O" film used sex and violence in a patently exploitative way, i.e. to inflame sadistic or prurient interest. A film could win approval, though, if it treated sex and violence-related themes in a way reasonably related to the honesty of the narrative, and in a non "sexploitation", non-psycho-sicko way.
It may raise a smile to see what was "banned" (awarded an "O" )40 or 50 years ago, but keep in mind that public entertainment had not turned into a total sewer at that time, and mothers and fathers were generally not willing to treat their children to sexual entertainment at a visual bordello. The League's ratings had the VERY widespread approval of Protestants, Catholics and Jews, including of course people highly aware of the sex-and-violence content to be found in both the Old and New Testaments.
Which could have been depicted with approval, of course, if handled in a non S&M, non-crotch-grabbing way.