NOTE ON USAGE: I mean by "theist" only a believer in some sort of god, as opposed to an "atheist."
As far as jumping to Catholicism, even if you ding her on the implausible authoritarianism of infallibility and her apparent disagreement over sexual ethics, all you are arguing for is to move her from Roman Catholic to Anglican.
I personally do not attempt to convert people away from Catholicism or pretty much anything else unless they ask me to or they initiate a discussion with me. I have numerous discussions with my Catholic father, but frankly wonder if I would not be doing a man well into his last 10 years of life a disservice by spending them with his son ridiculing his beliefs.
I would personally skip the Catholicism and Atheism thing. its just more likely to generate bad feeling than any great insights if you approach it as something you have to change about her. Actually, approaching any friend about anything as something you have to change about them is probably more a recipe for pain than for enlightenment, but I suppose reasonable minds might disagree over this.
Also, once you get all nuanced about religion and god and atheism, there isn't that much REAL difference between them, the difference is mostly labeling and signalling. Nuanced atheists believe they can be/are moral, just like theists. Nuanced theists understand that believing there is a deity and knowing details about her and what she wants and what she really thinks and how she really works (and even her gender, if any, for that matter) leaves them hardly better off than an atheist would be anyway at drawing a lot of actionable conclusions from this knowledge. Be gentle and interested in what she believes, and discuss things you genuinely want to discuss with her form that point of view. How she feels about joining such a hierarchical dogmatic religion, but not buying all the dogma. (Really not much harder than being proud to be an American while being against war, pro gay marriage, against slavery and against states' rights.) What is likely the case with intelligent alien races, do they get their own Jesus? Does god likely have different plans for different races? Would there be different moralities for different intelligences or would there be certain common things we can somehow derive from general features of intelligence? A lot of the same questions that were interesting to each of you before her conversion will continue to be interesting afterwards.
Indeed, I would love to talk to a SIAI-cognizant catholic about about simpler issues like cryonics, FAI and UFAI, and whether Jesus/God has visited other alien intelligences and will visit AIs once they establish a culture, or whether God would consider the AI to be such a natural "evolution" of humanity that he would count Jesus' visit as sufficing for both us and our successor species. What is god talking about when she talks about a soul? Can we build a soul, or rather can we build a machine (the brain is still a machine no matter what your religion) that has or gets a soul, or is there some secret miraculous sauce that god has to put in? And even if there is a secret sauce, would you not expect god not to touch any machine that we built that had the capability to support a good soul?
Believing in god doesn't relieve a person from contemplating the deepest mysteries of life and the world. It does not give a person an excuse to fall back to stupidity. The Catholic church includes the Jesuits and many other forces for knowledge and science and rationality in it. Which makes converts to Catholicism every bit as potentially interesting as converts to Atheism or Jews who spend hours a week studying the Talmud. Rather the opposite, it is probably hard to find a Jesuit who would love to go capo a capo against the best we have to offer.
Don't assume you know all that much more than she does. At least not without being willing to check.
Do you mean theists rather than deists about halfway through?
I recently learned that a friend of mine, and a long-time atheist (and atheist blogger), is planning to convert to Catholicism. It seems the impetus for her conversion was increasing frustration that she had no good naturalistic account for objective morality in the form of virtue ethics; that upon reflection, she decided she felt like morality "loved" her; that this feeling implied God; and that she had sufficient "if God, then Catholicism" priors to point toward Catholicism, even though she's bisexual (!) and purports to still feel uncertain about the Church's views on sexuality. (Side note: all of this information is material she's blogged about herself, so it's not as if I'm sharing personal details she would prefer to be kept private.)
First, I want to state the rationality lesson I learned from this episode: atheists who spend a great deal of their time analyzing and even critiquing the views of a particular religion are at-risk atheists. Eliezer's spoken about this sort of issue before ("Someone who spends all day thinking about whether the Trinity does or does not exist, rather than Allah or Thor or the Flying Spaghetti Monster, is more than halfway to Christianity."), but I guess it took a personal experience to really drive the point home. When I first read my friend's post, I had a major "I notice that I am confused" moment, because it just seemed so implausible that someone who understood actual atheist arguments (as opposed to dead little sister Hollywood Atheism) could convert to religion, and Catholicism of all things. I seriously considered (and investigated) the possibility that her post was some kind of prank or experiment or otherwise not sincere, or that her account had been hijacked by a very good impersonator (both of these seem quite unlikely at this point).
But then I remembered how I had been frustrated in the past by her tolerance for what seemed like rank religious bigotry and how often I thought she was taking seriously theological positions that seemed about as likely as the 9/11 attacks being genuinely inspired and ordained by Allah. I remembered how I thought she had a confused conception of meta-ethics and that she often seemed skeptical of reductionism, which in retrospect should have been a major red flag for purported atheists. So yeah, spending all your time arguing about Catholic doctrine really is a warning sign, no matter how strongly you seem to champion the "atheist" side of the debate. Seriously.
But second, and more immediately, I wonder if anybody has advice on how to handle this, or if they've had similar experiences with their friends. I do care about this person, and I was devastated to hear this news, so if there's something I can do to help her, I want to. Of course, I would prefer most that she stop worrying about religion entirely and just grok the math that makes religious hypotheses so unlikely as to not be worth your time. But in the short term I'd settle for her not becoming a Catholic, and not immersing herself further in Dark Side Epistemology or surrounding herself with people trying to convince her that she needs to "repent" of her sexuality.
I think I have a pretty good understanding of the theoretical concepts at stake here, but I'm not sure where to start or what style of argument is likely to have the best effect at this point. My tentative plan is to express my concern, try to get more information about what she's thinking, and get a dialogue going (I expect she'll be open to this), but I wanted to see if you all had more specific suggestions, especially if you've been through similar experiences yourself. Thanks!