Hi Benedict!
Bad news first: You will not be able to defend yourself. This is not because you're 18, it's not because you can't present your arguments in a spectacular fashion.
It is because noone will care about your arguments, they will wait for the first chance to bring some generic counter-argument, probably centering on how they will be there for you in your time of implied juvenile struggle, further belittling you.
And - how aggravating - this is actually done in part to protect you, to protect the relationship with your dad. With the kind of social capital, pride and identity that's on the line for your father, there is no way he could acknowledge you being right - he'd have to admit to himself that he's a phony in his own eyes, and a failure as a parent and pastor in the eyes of his peers.
To him it may be like you telling him he wasted his life on an imaginary construct, while for you it's about him respecting your intellectual reasoning.
Maybe the rational thing to do is not strive for something that's practically unattainable - being respected as an atheist on the basis of your atheist arguments - but instead focus on keeping the relationship with your parent intact while you go do your own thing anyways. Mutual respect of one's choices is great in a family, but it may not be a realistic goal given your situation, at least in respect to discussing god.
Good news: While this is such a defining issue for your father, is it a defining issue for you to tell your father publicly your new stance? How hard/easy would it be to let him continue with his shtick, retain the relationship, and still live your life as an open atheist for all intents and purposes - other than when with your family, where you can always act with mild disinterest?
Rational in this forum is mostly construed as "the stuff that works in optimising your terminal values". It is possible for you to be the "bigger man" here, depending on which of the above you value higher. But make no mistake - I doubt that you'll change anyone's opinion on god regardless.
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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.)