My Brother In Law was born a Catholic and went to a Catholic school and he supports Rangers.
He tells me even his teachers at school held a grudge towards him and wasn't allowed to play for the school team.
He is like most people i know in that although they were born either a catholic or protestant or went to Church or Chapel but actually have stopped practicing and religion is not part of their life anymore but it's the loyalties with friends and families that tie many to be 'born' into one club or another. The next generation through positive anti sectarian campaigns i think will have no fear to support either club no matter their religion. By the way there are a couple of kids in my boy's football team who are catholic and support Rangers so it is working already. ( just remembered there!).
As i've often said, the bitterness today which is still there (although to a lesser extent) will pale into insignificance in the next generation or 2 due to other fundamentalist factors.