This post continues my series of short answers to common (and not so common) questions about Catholicism. For the previous parts in the series, see the "Catholic Q-A Series" blog label.
My 25-year old cradle Catholic daughter is in a relationship with someone who does not believe in Jesus Christ. He has his own religion, similar to Judaism, but makes up his rules. She is now following him and renouncing her Catholic faith, and Jesus. What can I say, or do to bring her back?
Well, first of all, you can pray. I know, that's not a very comforting answer. We always wish there was more we could do. But, we have to believe that prayer is efficacious, that it really will help you and her. You must commit yourself to that.
Also, how comfortable is she with talking about religion? Most people who make this decision also don't like getting into it with their parents. Perhaps you could simply ask her: Why are you making this decision? Do you have any questions that I could help you with? Do you know why Catholics believe in Jesus?
If there are any doctrines of the Catholic Church that are sticking points for her, ask her if she would be willing to read some material that would explain these things for her. This is good because it takes some of the pressure off of you to have to defend what you believe, and it gives her the chance to learn about what the Church believes when she is ready (instead of right there, as she's talking to you). Remind her that there's no harm in reading something. Just hear the Church out, let Her have Her say. If she is fully convinced of the truth of her new faith, then it shouldn't bother her to read what the Catholic Church thinks.
Then, find a good book or an article online and give it to her. Your local Catholic bookstore owner can help you find what you need. I don't know what to recommend because I don't know what her needs are. Ask her to keep an open mind, and always be ready yourself to give a charitable defense of your faith when those rare opportunities present themselves. Whatever materials you find for her you should probably read yourself! You won't always have the perfect book at hand, but you will have your mind and heart, what you believe, why you believe it, your experience of Christ moving in your life.
Pray to the Holy Spirit that He will give you the proper words at the proper time.
Good luck to you!