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Atheists believe something can come from nothing.
The simple answer to this goes something like, "No we don't; we have no idea what started or preceded the Big Bang, and we're not claiming it popped out of nowhere. We're saying 'I don't know'."
The complex answer is exactly that: more complex. The entire concept of causality (philosophical shorthand for "cause and effect") rests inside time. There is an interval of time between, say, my dropping an apple and that apple hitting the floor.
We don't really know what causes time, what time really is, nor why immense gravity seems to have an effect on it. What we do know, however, is that time started with the Big Bang, so trying to discuss what caused it is really just a nonsense question at the moment
Really though, I shouldn't have to defend that fact that we don't know. The theist making this claim is really just saying, "The God I worship, which I haven't yet proven or demonstrated exists, can simplistically explain this problem, therefore It must exist." Or, in logical form:
- The Flying Spaghetti Monster exists.
- Part of the definition of the Flying Spaghetti Monster is that He can do X.
- X has been done.
- Therefore, the Flying Spaghetti Monster exists.
Even if you prove point 1, you'll still have to add a point in between 2 and 3 which reads: "X can only be done by the Flying Spaghetti Monster, exclusively."
Sorry theists, the burden of proof's on you. Logic's is a bitch, isn't it?
Atheists claim to know for certain there is no God.
No, actually, we don't. We believe there is no God. There's a difference.
Let's use Santa Claus as an example. Parents across the U.S. raise their kids with this pleasant and harmless idea that there is a chubby old man, dressed in red, who flies all around the world delivering presents to children, all in one day, who are good. He knows which kids are good, of course, because he's been watching them &mdash each and every one &mdash for the entire year, grading them with a remarkably simple "naughty or nice" system.
Kids always buy this story completely. After all, they're hearing it from their parents, who seem to know absolutely everything, and they keep hearing about it in commercials and storybooks.
As they grow up, however, they start accumulating extra evidence. Mom and Dad seem really tired on Christmas morning, Santa's handwriting looks like Mom's, they saw Mom putting presents under the tree, etc., and start to think, "You know? I don't think Santa Claus exists. I think it's much more likely that Mom and Dad made up Santa Claus just to make me be good."
Atheists know God doesn't exist in the same way that we all know Santa Claus doesn't exist; they've heard the arguments for and against the God hypothesis, and it seems infinitely more likely that God was made up to explain stuff that Bronze Age intellects didn't understand (like Zeus was for lightning) and to make people be good.