What Atheism Is
Some people think atheism is a matter of having faith in God's nonexistence. They think it claims god definitely does not exist, and that science has proven it. They think all kinds of ridiculous things that are false.
I do not personally know a single atheist that claims "god" is proven not to exist, or even that it's possible to do so in principle. Atheists say there is no evidence for god. Furthermore---speaking more generally---there is, by definition, no evidence for the existence of anything that really doesn't exist. In other words, for most things that really don't exist, you can't prove this fact; the best you can do is state, "There is no evidence."
People are superstitious, and atheists don't deny that. (In fact, it's what we fight against.) And, we don't dispute the fact that cultures have almost universally dreamt-up (different) gods either. But, argument "from authority" has no bearing upon what is philosophically obvious: atheism is the default state of a human. It is the "absence of belief in the existence of God or gods", or "disbelief in the existence of God or gods". Neither of these asserts the ability to absolutely disprove god(s). It's perfectly fair to describe a baby as having "an absence of god belief". It's certainly not fair to describe one as Christian, Muslim, Hindu, etc. (As Richard Dawkins has pointed out, it's just as stupid to call a child a Marxist or Capitalist as some religious title.)
Atheism is a "clean slate". It simply says, "You want to convince me? Show me the evidence!" It's exactly the attitude most people have about most extraordinary claims---and to which they blind themselves when it comes to their own particular childhood indoctrination. There are vast numbers of ridiculous and completely unsupported claims that some people really do believe with literally zero hard evidence: ghosts, fairies, goblins, telepathy, demons, pixies, thousands of different gods, dragons, elves, angels, zombies, telekinesis, gnomes, miracles (a HUGE category in itself), dwarves, voodoo, heaven, leprechauns, dowsing (e.g. water witching), eternal life, astrology, faith healing, remote viewing, prayer, alchemy, hell, crystal power, channeling, curses, and tons more.
Atheists simply recognize that gods have the same level of evidence as any one of the other things listed: none.
All the things we can possibly ever imagine either exist, or they do not. Is it rational to disbelieve something you've imagined, if there is no evidence at all for it? I say it is. Otherwise, we could never disbelieve anything.
The caveat is that new evidence may emerge, and therefore opinion may be modified. (This is called science.) Nevertheless, it is both rational and honest to disbelieve something that has no evidence. (In fact, it is the only reasonable response.) Similarly, it is both irrational and dishonest to positively believe something with no evidence!
Why Religion/God Fails
Religious people claim to be engaged in a different type of "seeking the truth". But, I don't accept that, and here's why: how can you "search" for something while having no possible way to verify or disprove any answer you "find"?
Searching for truth in any sort of meaningful way requires the verifiability or falsifiability of results---because otherwise, "search" becomes meaningless. (If you can't test the results, you don't know when you're done searching or if you're right or wrong anywhere along the way.)
And, what's the name of the system that generates "verifiable or falsifiable results"? Science.
There is no other way to know anything. Superstition doesn't cut it.
Oh sure, the religious are exceedingly fond of rambling on and on about the supposed virtues of faith. But, faith is not virtuous! It is the conscious decision to believe something is true, knowing full well that there is no good reason to do so. Or, as Dan Barker (an ex-pastor turned atheist) said, "Faith is a cop-out. It is intellectual bankruptcy. If the only way you can accept an assertion is by faith, then you are conceding that it can't be taken on its own merits."
"I contend that we are both atheists. I just believe in one fewer god than you do. When you understand why you dismiss all the other possible gods, you will understand why I dismiss yours." -Stephen Roberts