Go in panic mode.
This conference is not just making a case that Christianity is correct and debating about it. It's bombarding you with arguments for six days, where you won't hear an argument against Christianity or if you do it'll be awkward rude dissent from people in inferior positions, where you won't be able to leave or have time alone to think, and where you're going against your will in the first place. This is time for not losing your mind, not time for changing it. Don't keep an open mind, don't listen to and discuss arguments, don't change your mind because they're right, don't let the atmosphere influence you. If it helps you can think of it as like being undercover among huge patriots and resisting the temptation to defect (and their ideology may be better than yours), or like being in a psychiatrist hospital and watching out for abuse when you know the nurses will try to convince you your reactions are psychiatrist symptoms (and they may well be).
So don't see anything at the conference as a social interaction or exchange of ideas. Your goals are to get out of there, to block everything out, to avoid attention, and to watch sharply for anything fishy. Block out the speakers, just watch the audience. If there's a debate be quiet and don't draw attention. If you're asked to speak, voice weak agreement, be vague, or pick peripheral nits. If you're asked to participate in group activities go through the motions as unremarkably as you can. At the socials be a bit distant but mostly your usual self when making small talk, but when someone starts discussing one of the conference topics pretend to listen and agree, smile and nod and say "Yes" and "Go on" and "Oh yeah, I liked that part" a lot. Lie like a rug if you must. Watch the social dynamics and the attitudes of everyone and anything that looks like manipulative behavior. You'll be bored, but don't try to think about any kind of deep topic, even unrelated (doing math and physics problems in your head are ok, anything with a social or personal component is not). Try to get enough sleep and to eat well. Enjoy the ice cream. Don't think about anything related to the conference for a couple weeks afterward.
This is only short-term, and it won't help with your father; you probably want to handle that afterwards separately.
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A few notes about the community
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* Normal_Anomaly
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A note for theists: you will find the Less Wrong community to be predominantly atheist, though not completely so, and most of us are genuinely respectful of religious people who keep the usual community norms. It's worth saying that we might think religion is off-topic in some places where you think it's on-topic, so be thoughtful about where and how you start explicitly talking about it; some of us are happy to talk about religion, some of us aren't interested. Bear in mind that many of us really, truly have given full consideration to theistic claims and found them to be false, so starting with the most common arguments is pretty likely just to annoy people. Anyhow, it's absolutely OK to mention that you're religious in your welcome post and to invite a discussion there.
A list of some posts that are pretty awesome
I recommend the major sequences to everybody, but I realize how daunting they look at first. So for purposes of immediate gratification, the following posts are particularly interesting/illuminating/provocative and don't require any previous reading:
More suggestions are welcome! Or just check out the top-rated posts from the history of Less Wrong. Most posts at +50 or more are well worth your time.
Welcome to Less Wrong, and we look forward to hearing from you throughout the site.
(Note from orthonormal: MBlume and other contributors wrote the original version of this welcome message, and I've stolen heavily from it.)