I’m praying for you because of the enormous difficulty of this situation and I completely understand your desire to stay apart from this person. I especially pray for the child(ren) involved.
Why not be honest with this person? Just say “I’m working on forgiving you, but it is difficult because what you did to your child was so terrible that I no longer trust you. God be with you as you seek to change your life.” Or something to that effect (as upbeat as you can muster). By keeping your message brief, you will convey the fact that you do not want to be a part of their life. I don’t think you need to get into the fruits of his conversion, or a theological debate about the fundamental meaning of forgiveness (however I do think that what, Abby, Tantum, and Rob’s Wife has said is invaluable advice that should strengthen you in your resolve) or even discuss your relationship in any form with this person. Just take what he said at face value, send a short reply, and be done with it. Should this person have a genuine conversion, then you will see changes over the years. If it this conversion is simply a ploy to gain sympathy, then you have not allowed yourself to be manipulated. If you don’t want further contact, then if another letter appears, don’t write him back. Don’t even write back to tell him not to write. He’ll get the message. If he’s in jail, how can he be a part of your everyday life anyway? Will he be released in the near future? Has he corresponded with any other members of the family or does he see you, as a devout Catholic, as someone who might reach out to him when he is released from jail? Is there a chaplin at the prison that you might be able to talk to with out this man hearing about it? Or what about other relatives?
Sadly, most criminals never experience a real change and and the best place for them is behind bars so that they can not hurt anyone else. Remember, as a sexual predator, he will be very good at mind games and lies, and this could be one that he’s laying on you, so be very careful. Lastly, trust your instincts, they will serve you well.