“But doesn’t The Bible say, [insert passage chosen at random here]?”
This website provides verses at random. Bonus points if they have nothing to do with the situation at hand.
You might surmise that I’m lucky enough to have never had this problem. I might not be the best person to ask, but then, doesn’t The Bible say, “We are troubled on every side, yet not distressed; we are perplexed, but not in despair”?
I like it. 
There is a lady at work that I get along with really well. However, she is very religious (it’s just naturally come out in conversations) and she probably assumes that I am too, since it is the default around here. One day she’s going to ask me which church I go to or something, and I’ll tell her the truth, that I’m an atheist. I wonder what will happen after that?
I don’t. I don’t bring up the subject. If someone else does, I change the subject to something else.
A vote for this one. Short, to the point, polite, and an indication that you’re not willing to discuss the matter further.
That’s awesome. I’m imagining the dialogue going something like:
“But doesn’t the Bible say, [random, not relevant Bible passage]”
“What is that supposed to mean?!”
“It means ‘Mind your fuckin’ business!'”
Explain to them that you used to believe in God. Very, very devoutly. In fact God used to talk to you all the time. However since you started on the medication…
“I’d love to discuss this further some time, but right now I’m on my way to a meeting and I want first shot at the virgin”.
My sister is hyper-religious. She doesn’t know I’m an atheist but she knows I’m a doubter- I plead to Deist around her. The best argument I’ve found with her is that the apostle Thomas witnessed firsthand the miracles of Jesus- healings, walking on water, raising the dead- and heard firsthand from his mouth his sermons- and when standing face to face with the resurrected Jesus he still professed doubts until he put his hand in the scars. Jesus’s reaction:
While arguably evidence of frustration with Thomas’s doubts, this is far from a full-fledged rebuke and most certainly not a damnation. If Jesus did not condemn Thomas, who had far more reason than I do to accept the Gospels at face value, then surely He’ll understand the doubts of somebody born almost 2000 years later in another land who knows about it from 15th hand accounts. So, live by the Golden Rule/Ethic of Reciprocity and hope for the best.
It doesn’t satisfy her completely but it helps put her mind at more ease than before.
I like this one.
Judge not lest ye be judged - or some stuff like that - I’d have to look up the verse.
I suppose that telling the busybody that you’re worried about how loving God could be if shity people like [insert someone from the current news] are supposed to be made in his image wouldn’t go over well.
Ha! Unfortunately I got this:
Yep, quoting obscure Bible verses drenched with hidden significance at the nosy cousin-in-law sounds like a good tactic.
Or maybe something from the Koran.
Attacking her religion is probably not the best way to go unless she is being openly hostile.
Bonus points if she doesn’t notice it’s from the “wrong” book! 
I don’t defend not being christian.
I would, in response, ask them how they believe in a fairy tale. In one of the most deadly historical fiction fables on the planet.
That would be awesome!
Nosey Relative: “Ooooh I like that - what verse is it?”
MizQuirk: “Have a seat…”
Tell them that your beliefs are a secret between you and God, and you don’t discuss them with anyone else.
I consider myself a Christian and rarely discuss my beliefs. If ever pressured, I pull this out.
How would y’all react if I started a thread titled “How do you defend being a Christian” and a lot of the advice was on how to hide the fact or change the subject?
Happened to hear an old German song the other day.
Paraphrasing the lyrics:
Everyone wants to go to Heaven, but nobody is in a hurry to die to get there.
I usually just do my best to avoid the subject - but if someone insists on bringing it up I just tell them there are lots of religions to choose from, but another choice is not to choose any of them. That is my choice.
However, I do like some of the more blunt comments above…
Uh, who’s hiding, in the context of this thread? Said relative already knows that the OP is atheist. The question is about how to "defend’ against the “but why aren’t you Christian” questions, and I just can’t see that the relative has any inherent right to that information. Plus, giving her a reason of any sort just gives her an opening to try to counter your reason and tell you that you’re wrong. No reason = nothing to counter.
Why would I change the subject? Because I just don’t see the point in engaging in a long, drawn-out argument that I already know is just going to go in circles, and neither one of us will ever change our minds. I have better things to do with my time. YMMV.
It’s okay to keep some things private. It’s not some sort of moral failing to do so. I don’t discuss religion or politics (or really, anything personal) at work. I don’t discuss my sex life with anyone who’s not directly involved, my therapist, or maybe a confidant-level friend. I’m not “hiding” my sex life, or my religion, but I don’t feel a need or obligation to share with people for whom it’s none of their damn business. I don’t need their approval to be who I am.