Surely the only valid response is laughter?

I just laugh. I can't say it won't happen but doubt it so strongly as to ridicule this God of theirs. I tell them that I hope to actually have that Judgment Day so I can look Thou in the eye and ask what kind of sick motherfucker Thou is? Was the experience I just had to endure really necessary or some kind of uncomfortable joke? That usually shuts them down but gets a horrified chuckle.

"if you dig a pit for someone, you end up in it yourself"

Sad that their religion is based on reward and punishment. The only hell is theirs.

If Pope Francis says I can get into heaven as an athiest, just trying to do good and living by my own moral code, then that's good enough for me. As a Catholic, I would be expected to repress my sexuality. As an atheist, I don't have to.

And the scoreboard says:
Atheism 1 - 0 Catholicism

Ok, see you soon.

"Demons, sodomy, and so much heat the clothes are likely to come off. Doesn't sound too bad to me."

Such a threat has about as much meaning to me as if they'd exclaimed "bopity bop plobalop wibble wobble woo", although slightly less amusing.

I generally just shrug and ignore, unless in the mood to have a lengthy and tiring debate where I request objective evidence for the claim and then have to exhaust myself explaining how citing a religious text as proof of it's own accuracy is a circular argument and not an objective one.

Disbelief, and an uncontrollable smile

It’s actually difficult to recall when I’ve been ‘threaten’ like that … and it generally came from the craziest of the crazies. One religious nut wearing a sign-board and using a ‘bull horn’ in downtown Portland (Ore) as my young daughters and I waited for a light rail train didn’t like my questioning his rant... It was a good experience for them to witness the base hatred in such people.

The other was after my mother and I stopped to take on the religious crazies picketing an woman’s health clinic, also in Portland. We’d worked them down a side street… and when they finally realized what we’d done, really got pissed! Kept em out of the public’s eye for maybe a half hour We’ve some serious ‘hell time’ if they’ve any pull! But Mom’s always said, ‘she wants to end up wherever Pat Boone isn’t!’

I tell them to kiss my Agnostic ass...and if there was a hell my fare is all paid for!

They don't get it. Nothing you say hits them in the brain, the wilder the religious story is the better. It's insane.

My standard response has been to tell them they're going to suffer in Tarturus for forsaking Zues and the Olympians.

I don’t care enough to respond to them

I try not to laugh, as I don’t feel the need to be disrespectful, so I try to smile and say something in line of ‘We shall see’, or ‘That is not your concern’.

On the rare occasions when that has happened, I push the question back to them and ask if they think that I deserve to go to hell. They will often try to avoid the question by saying that it is not their decision and "only god can make that decision". I then ask if they agree that their god would send me to hell for using the 'free will' I have been given to not believe. Watching the cognitive dissonance and grappling for nonsensical excuses is entertainment of the highest order.

Well, if it is a serious, respectful conversation, I try to inform them that the concept of Hell was implemented as a control feature, and try to point out the obvious contradictions between a loving God and a God that sends frail, fallible primates to an eternal deep fryer.

Threatening an atheist with hell is like threatening an adult that they won't get presents from Santa. I just laugh and walk away.

Mark Twain's "Letters from Earth" comes to mind. Why would I want to be surrounded by stupid people, who can't sing or play a musical instrument, try to do both to praise God? That cacophony alone would be Hell.

I think I'd rather be where all the philosophers, scientist, and interesting people are.

Usually, laughter and a "May the Farce be with You"

In certain circumstances, I might reply with "You mean with 'Satan-?', oh, I'm not worried about 'Satan', he's my squirty-wirty little slut-buddy, no doubt about it-!" or some other thing that comes to mind in that moment.

"Been there, done that."

If heaven exists it's filled with the pure and boring people. Hell will be where the party's at.

If I'm told I'm going to hell, a simple "I'll see you there, I know you've eaten shrimp" is my usual responce

I leave the room.

There's not really any point to discussing how illogical and erroneous that person's position is, so I might say, "Oh. Good. I'm looking forward to being with my family."

They can't threaten me. They don't have the power to punish me. The idea that people can be dealt into two places (like red and black cards) is ludicrous. We are all between, all shades of gray.

I point out the following: If a man is a pillar of the community, a wonderful son, father, husband, neighbor, etc., a decent and honest person, yet although he has heard about Jesus he has continued to follow the faith of his family, ancestors, and community, then the bible would condemn him to hellfire for eternity. On the other hand, a man who is convicted of raping and torturing people, if he is a serial murderer condemned to death, then on the eve of his execution he converts (and being so close to death, this may be a sincere conversion) and thus is assured a place in Heaven for eternity, then this standard is absolutely ridiculous. I don't see how anyone could worship a god who would decree that this is what happens.