If you tell me you have a dog, there's no reason not to believe you - and if it isn't true, so what? However, if you tell me you have a talking dog, I'm going to need more than just your word for it! Extraordinary claims require extraordinary evidence. No religion has ever offered anything remotely close to extraordinary evidence for any of their gods or any of their extraordinary claims - and that's a fact. If any religion ever had done so, it would be the only religion operating today, having debunked all others, putting them out of business. Nobody would be arguing about the existence of God, and there would be no atheists. But none of these things are true - and that's the proof. Any claim that some sort of "supreme being" exists outside of space and time is by its own definition unknowable and un-provable. Anyone who claims to know something which can't be known is either a fool or a liar or both, and arguing for something which is unknowable and un-provable is indistinguishable from arguing for something which doesn't exist!
All religions claim their god exists – a claim which is impossible to prove, and for which no evidence exists or even could exist, minus a personal appearance from said god. Any so called "personal god" who supposedly interacts with us would leave evidence of his interaction – no religion has ever produced any such evidence. Some claim that "holy books" like the Bible are evidence, but not only does the Bible not even come close to being extraordinary evidence, it isn't even good evidence. Everything in it is folklore, second hand or worse storytelling, and hearsay – the weakest kind of evidence there is. There are no original copies of the books of the Bible, and nobody really knows who wrote them or when. And to make matters worse, the Bible has been copied and recopied many times, over many centuries, translated into many languages, and some parts have even been forged. To assign any authority to books like the Bible is an affront to intelligence and common sense, and any real attempt to determine truth. Even today, eyewitness testimony is considered to be highly unreliable – it's just somebody saying something, like claiming to have a talking dog. Without corroborating evidence, it doesn't mean anything, especially if the claim is extraordinary.
Either "God" exists or he doesn't – it's a black or white proposition. The positive claim that God exists is an extraordinary claim which requires extraordinary evidence, and the person making the claim has the entire burden to prove it with evidence. Nothing any religion to date has come up with even comes close to good evidence, much less extraordinary evidence. And that's why atheists reject the claim. Atheism isn't a "world view" or a religion or a belief, and it's not based on faith. It's a single issue position – rejection of the existence of God due to lack of sufficient evidence to support such a belief. Atheism is the most rational position to take, and that's all there is to it. Pretty simple huh?

Many religious people try to claim that the mere fact that we exist proves beyond all doubt that the universe has been "finely tuned" just for us, by a "creator deity" and that nothing else could possibly explain it. There's no way all the intricate elements and attributes which make life possible for us could have happened by accident or by random chance, thereby proving the existence of "God." I'm sure you've all run into people like this. They say there cannot possibly be any other explanation for the fact that we live on just the right kind of planet, just the right distance from just the right kind of sun, with a moon just the right distance away, with just the right kind of atmosphere and the perfect range of temperature to support life, and so on and so on. To believe that this is all some kind of coincidence or random accident based on pure luck is totally illogical, and anyone who proposes it could have happened without "God", like a scientist for instance, is either stupid, lying, or working for Satan.

Of course this is an irrational and invalid argument because the conclusion is simply the premise restated... A = B therefore A = B. Not much of an argument is it? But they claim the finely tuned universe argument is "pure common sense" despite the fact that in an essentially infinite universe an infinite number of possibilities would have to exist, including forms of life that we may or may not recognize as life, formed in ways we may or may not understand. Without a time machine which can also transport them to every macro and micro sized area of the universe all at once for inspection, there is no possible way to know what's out there. And it should be painfully obvious that unless they can also describe in exact detail what a universe that has not been finely tuned by a creator looks like for comparison, claiming that the one we live in is unquestionably fine-tuned by God is a completely un-provable claim based on nothing. But religious people already believe illogical nonsense based on nothing, so they're incapable of understanding simple logic like this - otherwise they wouldn't be religious to begin with, or arguing with you in the first place.

So here's a way to cut to the chase and nip arguments like this in the bud - pay attention, it goes by pretty quick. When someone claims that the only "God" could fine tune the universe to the point where we could exist, ask them this question: "So you're saying that if things weren't the way they are, they'd be different? That's your argument?" This is one of the dumbest, most absurd arguments for the existence of God I've ever seen - it's right up there with "God wrote the Bible and the Bible says God is real, so God is real." Anyone who's this stuck on stupid is a waste of time to bother with. Just ask them the question, have a good laugh at their answer, and go on your merry way. And try not to be too bummed out that we still have to deal with some of our fellow humans who were designed so poorly that they actually still believe absurd crap like this in the 21st century. Religion is a cancer on society - it rots the mind and this kind of thinking is just yet more proof it does, like we need any more proof.

RELIGION IS A CANCER ON HUMANITY! It can be disturbing at times to see just how brainwashed, ignorant and superstitious some people still are in the 21st century, especially creationists. Some creationists actually say that God made light go faster at creation so Adam & Eve would be able to see the stars he had just created, because this allows them to ignore the speed of light which proves the universe is billions of years old, not thousands as they claim. If you've ever debated one, then you know the only way they can defend their claims is to categorically deny science. This makes debating them frustrating because it's hard to make points with someone who's delusional and completely out of touch with reality. The good news is their beliefs back them into a corner which you can easily exploit.
Pascal's Wager is basically an argument that it's better to be safe than sorry - an apologetic argument proposed by the seventeenth century French philosopher Blaise Pascal. In a nutshell, Pascal proposed that it's better to believe in God than not to because if you're right you're saved, and if you're wrong you've lost nothing. Of course the huge flaw in this argument is that an all knowing God would know that you're just covering your ass and faking your faith, so it falls apart on its face.
The speed of light proves conclusively that the universe and the earth are billions of years old because of the time it takes light to travel great distances, which for most sane, educated people completely debunks the creationist claim that the universe and the earth are only a few thousand years old. The problem is, when you're dealing with a creationist, you're not dealing with a sane, educated person! The best and only argument a creationist can make against the scientifically measured and proven speed of light which confirms the age of the universe is to claim that God has somehow monkeyed with light for some reason to make it look like the universe is billions of years old when it really isn't.
Now of course such a claim is completely un-provable unless God makes a personal appearance to explain why he's been messing with us. And of course it goes without saying that any claim made without evidence can be summarily dismissed without evidence. The only way creationists can argue that the universe and the earth are only a few thousand years old is to believe that God has tricked us. To be a creationist you have to believe that God purposely deceives us by tampering with the conclusions of science as some sort of way to test our faith or something. To be a creationist you must believe that God is a trickster.
So here's how you corner your creationist opponent using their own argument. Make them go on record stating that they believe as fact that God deceives, us for whatever reason, and that the conclusions of science are false because God has the power to make things look a certain way when they really aren't. Once they've committed to this position, ask them this:
"Now that you're on record as saying you believe God deceives us on purpose, how do you know with 100% certainty that God didn't write the Bible as a trick, as a way to weed out really gullible and stupid people who refuse to use the marvelous and powerful brain he gave us - the kind of people who would believe that a book of obvious fairy tale nonsense is a true account of history and description of the nature of the universe, without any skepticism or need for any evidence whatsoever? How do you know with 100% certainty that you aren't condemning yourself to hell in a reverse version of Pascal's Wager by believing the Bible is a true historical account, and not a trick by God to test you to see if your brain is functioning correctly? How do you know that it isn't us atheists who are God's chosen people, because we have the ability to tell fact from fiction because our brains are running on all cylinders as God intended, and yours aren't? This theory is in 100% agreement with your claim that the Bible is the word of God, so there's no disagreement there, and the entire creationist argument for a young earth must include God playing tricks on us so there's no disagreement there. The only difference is what his intent was in writing the Bible, which of course only God could know. So... since you are not God, how can you possibly prove my theory is wrong, especially since I've given you exactly the same amount of evidence for mine as you've given me for yours - none whatsoever.