1. You Can Do Terrible Things in the Name of Either One
We're putting aside the question of which belief system has killed more people by percentage of population, or whether a hypothetical world without religion would have seen fewer or more genocides than ours. We're not going to open a spreadsheet and try to count which belief system manufactures more murderous sociopaths per capita.
All I need from you is agreement that it's entirely possible for either an atheist or theist world to devolve into a screaming murder festival. The religious leader sends his people into battle because he thinks God commanded it, the Stalins and Maos of the world do the same because they see their people as nothing more than meaty fuel to be ground up to feed the machinery of The State. In both cases, the people are equally dead.
Yeah, yeah, I know the Christians are saying that the guy who fights an unjust or needless war is violating God's law, and thus isn't a good Christian. Meanwhile, the atheists are saying that Stalin was merely bloodthirsty, separate and apart from his disbelief in a higher power. Both believe, then, that it is a corruption of their belief system that allows unjust slaughter to happen.
But for this project, All we need to agree on is this: it happens in both cases. And if the opposing belief system vanished tomorrow, war and bloodshed and terror would still take place.
And can we further admit it's actually physically impossible to calculate whether, if your side had its way, the volume of terrible things happening would go up, or down, or stay the same? I know you have an opinion on that, and I can guess what it is. But we don't know, and can't state it like it's fact. Right?
Everybody still on board?
Good. Can we now also agree...
2. Both Sides Really Do Believe What They're Saying
Christians do this thing that drives atheists nuts, where they talk like God is patently obvious to all mankind, and that atheism is therefore just petty, intentional rebellion against Christians. In other words, that atheists don't honestly believe what they say, and just say it because they're jerks.
But atheists do something very similar, particularly when a Christian says:
"Only the saved go to Heaven!"
...and what the atheist hears is:
"I want everyone else to go to Hell!"
It's the same thing, thinking that deep down Christians don't really believe this is the law handed down by a creator, and therefore Christianity is just a petty, intentional rebellion against the non-Christians of the world. In other words, that Christians don't honestly believe what they say, and just say it because they're jerks.
But all that is just a way to make cartoon villains out of the people who disagree with us. And if we stop and think about it, we'll see it's asinine.
Atheists, you know that Christians have freaking died because they refused to walk away from what they believe. That goes beyond simple human stubbornness. I mean, I can tell you first hand. I was raised in a Pentecostal church (like the one they visited in the Borat movie).
I soured on the whole religion thing in my teens, as you can probably imagine, and then came back to it later kind of on my own terms. From that experience I can relay this fact: If there's no God, then there is something in the human brain that can and does present an amazingly realistic impression of one. A gland, an artifact of environmental pattern recognition, whatever you want to pin it on, the result is, at certain times and in certain moods, as tangible and real and distinct as the person sitting across from you on the subway.