Well let me say from the voice of experience, and I don't mean this in a 'but in' way:) Don't deny them the right to speak about their beliefs with you. My dad has told us that if we try to talk about "religion" or the bible with him there will be a fight. He doesn't want to hear it. The priest tells him what he needs to know. So consequently we are not real comfortable around each other, because, have you noticed how many subjects can lead to that type of conversation:) which makes our visits with each other rather stilted. Besides if he was wanting to change me back to "catholic" (which is in my opinion, if he thinks his way is the truth, he should be wanting to do) how is it going to happen if he won't talk.
I believe part of the reason he won't discuss it with ANYONE is because he has been confronted with Scripture at sometime and has seen stuff that doesn't line up with his knowledge of what catholics believe but doesn't know enough about the catholic faith to defend it, and he doesn't like feeling ignorant of something that he has spent his whole life professing belief in. It is really very sad, but I feel is the condition of alot of people on both sides of the coin. People have been so inundated with the things of the world that they don't think they have time to study the bible each day for themselves, so trust that the priest or preacher in their church WOULD NOT lead them astray.