This justifies weak atheism (lack of belief in gods) but not strong atheism (belief that there are no gods). Lack of evidence for a proposition ("God exists" in this case) is, in and of itself, not evidence that the proposition is false. However, lack of evidence for a proposition combined with the expectation that if that proposition were true that evidence would be available does constitute evidence that the proposition is false. As an example of this reasoning, suppose someone claimed that there is a herd of invisible two-ton elephants stampeding through your living room. If such a claim were true there would be plenty of evidence in the form of broken furniture for example. Now you examine your living room and find no evidence for stampeding elephants. It is, of course, rational to believe that the elephant claim was false. Now lets consider the gods of the dominate theistic religions: Judaism, Christianity, and Islam. Each of these religions postulate a god that is concerned with human welfare and that performs miracles. If such a god exists, there should be ample evidence of the miracles that he works whenever human suffering is present. But human suffering certainly exists and there is no evidence of the theists' god. This constitutes evidence that this god does not exist.

The fact that evil and human suffering exists is strong evidence that the theists' version of god, which is suppose to be both benevolent and omnipotent, does not exist. Of course, theists have rebuttals to this type of argument, but they are very strained and totally unconvincing. Does anyone honestly believe that "sin" and "free will" can explain the natural disasters that kill so many people every year?

Another reason for believing that the theists' version of god does not exist is the Argument from Nonbelief which goes as follows: The theists' god is a benevolent god who rewards believers and damns nonbelievers; such a god would want everyone to be a believer. Since the theists' god is also omnipotent, he should easily be able to convince nonbelievers to believe. It then follows that if the theists' version of god exists, there would not be any nonbelievers. There are, however, many nonbelievers in the world. Therefore, this god does not exist.

What about a more generic god? Suppose that there is a conscious god that created the universe and now just stands back and watches his creation without ever interfering with it. Ockham's Razor (don't postulate the existence of anything more than what's needed for explanation) certainly justifies weak atheism in this case. The basic idea behind Ockham's Razor can be used to justify belief in the nonexistence of this god as well. Since we cannot know for certain how the universe was created or even if it was created, the best we can do is decide if some of the many possible scenarios are more likely than others. Applying the idea of Ockham's Razor, we have to conclude that the scenarios that involve a more complex explanation for the universe are more unlikely than the scenarios with a simpler explanation. Since explanations that invoke a conscious god include an extremely complex being and naturalistic explanations do not, we can conclude that the scenarios involving a god are much less likely than scenarios that do not. Therefore a tentative belief that this generic god does not exist is justified.

For more arguments for the nonexistence of gods, see the Secular Web's Arguments for Atheism.

In conclusion, weak atheism is the default position. If there were no evidence for the existence or nonexistence of gods, weak atheism would be the only rational position to take. In order to move from the default position of weak atheism to theism, a rational person would require credible evidence for the existence of a god. Similarly, to move from weak atheism to strong atheism, credible evidence for the nonexistence of gods would be required. Some freethinkers find that the evidence that gods do not exist is convincing and that strong atheism is justified. Others think that the theists' description of their god is incoherent so that "God exists" is unintelligible and "not even wrong". Many atheists are also agnostics; even many strong atheists claim that the available evidence is sufficient to justify only a tentative belief in the nonexistence of gods and that actual knowledge is impossible. Finally, it should be noted that the position a person takes can vary depending on which version of god is being discussed.

If reason dictates that we should not believe in gods, then why do so many people still believe? One answer is memes. Another answer is close-mindedness. Many theists were brainwashed from birth to believe in their religion and they are simply not open-minded enough to ever seriously and honestly question their beliefs. Many freethinkers in the USA, on the other hand, were raised as theists but were open-minded enough to question what they were taught; the result is disbelief.