During my time in college, one of the strategies I employed to bring the Gospel to my classmates was to wear attention-grabbing Christian t-shirts. That way, they start the conversation. One of the shirts said "You need Jesus bro. Just sayin'". This was my favorite, and it turns out it was also the most effective. One of the daftest atheist girls I've ever met (we're talking the "God can't be real because His name's dog spelled backwards" type) absolutely loved it the first time she saw it. She loved it so much that she drew the attention of the whole corridor to me by shouting it out loud.
One of the boys stood next to me said "what if you're an atheist?" This is one of those moments where I thought about my response a bit too late (seriously, Christians, when Peter said always be ready, he meant it...). What I wish I'd have said is "that won't make Hell a single degree colder." Unfortunately, all I said was "you still need Him." True, but hardly a memorable conversation that will stick with him forever.
But to you atheists reading this, I don't need to be quick witted. I've got all the time I need. And so, to you, I can say this: Your unbelief will not make Hell one single degree colder. That's not a threat. It is a fact that our beliefs, or lack thereof, have absolutely no effect on the outside world. Now, you can live as if your beliefs are true. "Don't worry, I don't need to brush my teeth, they won't rot." You can live as if your teeth won't rot. But if you don't brush them, they will rot. Then everyone who "threatened" you with rotten teeth won't seem so crazy.
Most atheists will acknowledge the fact that they're going to die. But just as you can't control what happens during your life through belief, you can't control what happens afterwards by belief.
Except, that's not entirely true. While disbelieving in God does not make Him not exist, believing in Him does change how He will treat you. If you believe in the death and resurrection of Jesus, you cannot be condemned. So, as my favorite t-shirt says, You need Jesus bro. Just sayin'.