I will face a similar situation when I get married, because my mom raised me Catholic and I no longer identify as Catholic. I've thought a bit about what I will do when that day comes.
I would not argue this in person, but rather write a letter. In the heat of the moment not many people are willing to change their religious views, but if they have time to think on it, they can come around.
1. Use Catholic beliefs to your advantage. I don't believe the Vatican thinks that Protestants will be condemned to hell. Catholics think that Protestants got it wrong, but still believe that they are Christians.
2. Use arguments from Christian scholars who are not Catholics. This is a fine line to walk, because you don't want to diminish your moms faith, but try to convince her(with sound scholarly arguments) that you are just as saved as she is, and you don't have to attend the mass to be saved.
Good luck. My mom will be absolutely devastated when she learns I'm not getting married in the Catholic church, and equally devastated to know my kids won't be raised Catholic. It's your life though, and if she can't accept your decision in this life, at least you know she'll be there for you in the next.
This post was edited by IceMage on Sep 14 2014 06:19am