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Isn't saying "There is no god" logically equivalent to saying "There is a god"?
Where did you learn logic?
Let's talk about the standard of reasonability:
A hypothesis is reasonable only if it is testable, that is, only if it predicts something other than what it was introduced to explain.
Other things being equal, the most reasonable hypothesis is the one that is the most fruitful, that is, makes the most novel predictions.
Other things being equal, the most reasonable hypothesis is the one that has the greatest scope, that is, that explains and predicts the most diverse phenomena.
Other things being equal, the most reasonable hypothesis is the simplest one, that is, the one that makes the fewest assumptions.
Other things being equal, the most reasonable hypothesis is the one that is the most conservative, that is, the one that fits best with established (scientific) beliefs.
We should accept an extraordinary hypothesis only if no ordinary one will do.
Belief in God not only is untestable, but in fact defines God based on faith, and therefore defies testing. (see Babel Fish).
Do events around the world comply with what we could expect if there were a God (especially a benevolent one)?
How many assumptions does belief in God require, compared to disbelief?
Which belief flies in the face of more well-established scientific knowledge?
Most people who call themselves agnostic mean that they don't feel there's enough information to make a decision as to whether or not God exists (frequently because their making an fallacy; thinking that because it hasn't been disproven that it must be worth consideration. Under the standards of reasonability I went over before, it's clear that god-belief is quite unreasonable. Believing in God doesn't profit you anything intellectually; in actuality, it creates cognitive dissonances which prevent fruitful intellectual thought.
Once we come to the conclusion that belief in God is unreasonable, we can discount it and begin the pursuit for reasonable explanations for the things around us. That's why I don't believe in a god: because you can't learn anything that way.
As for why I (and other atheists like me) argue the point with theists: first, it is to help establish and solidify our own beliefs, by forcing us to defent--and thus think about--them; second, it is because of what we call "positive atheism," the belief that theism is morally, intellectually, and socially destructive to humanity. Think about how many wars have been fought over religion. And while 10% of americans profess no religious belief, only 1% of prison inmates do. When was the last time an atheist bombed a clinic or hijacked an airplaine?
I think that should just about do it for now.