polite respose? nevermind, the best I could do is passive agressive
The thing is, you’re asking how to respond politely to a rude question. There is no perfectly polite way to tell somebody they’ve crossed a line. I think what you already said – that you simply weren’t going to discuss it with her – is already perfect.
I have a brother who’s a born-again Christian, and I, along with my other brother who’s an atheist like me, have a long standing policy when we talk that we’re not going to talk about religion, period. We don’t give him a hard time, he doesn’t give us a hard time. We talk about sports, movies, cooking, even politics, but we all know religion is a third rail so we just don’t go there.
My usual strategy is “thank you for sharing” – full stop.
**So here’s my question: *****How would you respond (politely) to someone who feels compelled to chastise you about spiritual things? ***
I’d probably offer to sacrifice a goat to Ba’al in their name so that they’d quit interfering in business that was not their own.
If it was someone I didn’t care about I’d tell them to go fuck themselves and get over their stupid superstitious beliefs.
If she’s going to keep insisting on inserting her judgmental comments where they are not welcome (which leads, of course, to the question: why the fuck is this person a friend? but that’s not the subject of the thread), you could do the opposite of trying to shut down the comments — you could invite them, with cheerfully veiled sarcasm.
“So, since it’s so important to you to tell me how you feel about all of my life choices, I’m going to give you the full rundown on everything you should judge me on. Let’s start with foreplay. I really like sucking his dick. I run my tongue all up and down the shaft, and I cup his balls. I can make him come almost instantly by pressing on his taint, so I tease him with gentle strokes, until he’s like groaning and begging for release. Which sin is that? … Okay, good. Now, let’s talk about anal.”
See how long you can drag it out before she gets the hint.
I just tell people that my relationship with God is between me and him, and unless they want to start speaking for him I’d appreciate if they would leave it up to him.
I like asking “Why is this so important to you?”
Assuming you’ve not been married previously, you could say you plan on getting married just once and if it’s going to last the rest of your lives there’s really no hurry.
Or that you’re putting it off so the married part won’t seem so long.
I usually just laugh at them. Not a guffaw, just a chuckle, as though I’m saying they are a silly person. She will eventually get tired of being giggled at.
Fart. Rudeness begets rudeness.
Failing that, silence.
If it keeps up, as Diogenes said, what you’ve done is fine.
It depends if the person doing the chastising is coming from the same spiritual position you’re in. I can avoid Christian chastisement by saying I’m Jewish, which gets around the whole atheism bit. If there ever had been Jewish chastisement I could do the atheism bit. (There never has been.) And atheists are too polite to do it. 
So, maybe a “my beliefs are different now from where they used to be” would do it for most people.
I like this quote two. It goes well beyond my less than diplomatic first thought.
[Berta Mode]“Back Off, Thumper, before I knock the John 15:7 right outta your keister…! I’ll verily do unto you until your own Savior wouldn’t recognize you.”[/Berta Mode]
To the more abstract question, pretty much the way you’ve been doing. Thank the person for their concern for me. Repeat, firmly, that I’m not going to discuss the subject with them (this extends to me as well, I can’t open the door on the topic, either). And then find something else to talk about.
I think it also matters whether or not you’re still part of the same religious community as the other person. In general, I think that people do have some (very small) level of responsibility to other people’s well being. Even if it’s just saying “Don’t drive off the cliff” when you see someone driving toward a cliff. Within a shared religious community, I think that the responsiblity extends to each other’s spiritual well being as well. And in that context, there are times where I think spiritually based advice is appropriate. Still it ends with thanking them, declining to discuss it, and changing the subject.