When trying to escape the burden of proof, atheists will often say "you can't prove a negative". In a sense, that's true. First off, this doesn't relieve the atheist of the burden of proof, as they would like. You just can't say "sorry, my worldview is impossible to prove, so you must accept it as absolute truth". But also, it actually demonstrates the absurdity of their confidence.
No matter how intelligent a man gets, he will never have infinite knowledge. He will be dealing with an extremely limited amount of knowledge, a lot of which will actually be false. For example, myths like blood being blue while inside the veins still persist. Even college teachers (including my own) continue to profess it, yet a simple trip to the doctor will demonstrate otherwise.
Limited knowledge, false beliefs, and, on top of that, bias, all make it impossible for any rational person to completely dismiss the existence of God. Let's have a look at my fruit salad analogy. The fridge represents the atheist's brain, and every item within represents a fact or a claim about the universe. The fruit salad represents religion, and apples specifically represent the Judeo-Christian God. The atheist's fridge contains many things. Beer, milk, ham, cheese, all sorts of stuff like that. But in all of that, there is not a single fruit. He therefore declares "fruit salad is a myth! Only idiots believe in fruit salad!"
But a Christian turns up and places a fruit salad in his fridge. The next day, the atheist finds the fruit salad. But the atheist messes with the fruit salad a bit. First, he declares "pfft, grapes are vegetables!" Here we have an example of a false belief preventing him from believing what's really there. Then he examines the salad a little closer. There are a total of 4 fruits in the salad (three, if the atheist insists on counting grapes as vegetables), but scientists say 5 a day is healthy. "Your evidence is incomplete!" He yells. Then he adds a tomato, which, while still a fruit, obviously does not belong in a fruit salad. "Your fruit salad is disgusting!" He yells. This represents attributing traits of one religion to Christianity. For example, Islamic terror attacks have nothing to do with Christianity, but atheists use them to dismiss all Theistic religions collectively. He notices that the salad lacks pineapples, and says "it would be a pretty awful fruit salad without pineapples!" This is like when atheists say they don't like Christianity because it doesn't believe what another religion does. Finally, having examined all the fruit salad, he picks up an apple and takes a bite. "Eww!!!" He shouts. "I hate apples!" This represents the atheist's bias. There are parts of Christianity that atheists find undesirable. Difficult doctrines, especially those which condemn the atheist's favorite sin, are a huge put off.
The atheist, having completed his analysis, concludes that if this is a fruit salad, he doesn't want it in his fridge. He hands it back to the Christian and continues in his denial of fruit salad, happily munching away on the expired ham he's had for years.
Here we have a picture of atheism. At first, the atheist bases his dismissal on a lack of information. Then, when information is presented, it is rejected, not because it does not exist, but for a variety of reasons, such as misunderstandings, and even bias. In the end, atheism is a very unwise philosophy, akin to checking your fridge, finding no fruit salad, and declaring apples, therefore, do not exist.