Atheism, as it exists today, is not a belief system.
Until you have reason to believe any given thing exists, you must assume its nonexistence.

The very nature of assumption is a belief system. There is no proof for the thing assumed, there is nothing in the scientific method that states that that if there is no proof for A, then not A must be assumed. Assumption is belief. Webster1913 even does a good job describing belief:

Assent to a proposition or affirmation, or the acceptance of a fact, opinion, or assertion as real or true, without immediate personal knowledge; reliance upon word or testimony; partial or full assurance without positive knowledge or absolute certainty; persuasion; conviction; confidence; as, belief of a witness; the belief of our senses.
In every sense of the word, an assumption is a belief.

To disprove the existence of God, there must be similar evidence of non-existence. Proving the existence of anything gets into very tricky (and either very interesting or non-interesting depending on your philosophical bend) metaphysics. There is an entire branch of philosophy that believes that I am the only mind in existence - its called solipsism. Arguing with such individuals is a complete waste of time - they believe (many claim to have proof) you to be a figment of their imagination - as is everything else.

The burden of proof is always on the person making an assertion or proposition. Shifting the burden of proof, a special case of argumentum ad ignorantium, is the fallacy of putting the burden of proof on the person who denies or questions the assertion being made. The source of the fallacy is the assumption that something is true unless proven otherwise.

From Black's Law Dictionary:

The obligation of a party to establish by evidence a requisite degree of belief concerning a fact in the mind of the decision-maker in order for that party to prevail that fact.

(From dizzy's writeup in Burden of Proof, links are mine)

To make any statement, the burden of proof is upon the individual making the statement to prove that it is such. With any skeptic worth his or her weight in ideas, assumptions are forbidden. The quest for knowledge is not over when it has been shown that 'A cannot be proved', and certainly it does not follow that 'not A must be true'. This is the credo of skeptic - academic skepticism and Pyrrhonism both agree upon this point: The skeptikoi continues to search.

Given the evidence at hand, the belief in a God or gods is just as reasonable as the belief in the non-existence of these entities.

The pursuit of truth does not and cannot end with 'it is impossible to know'.

The essence of If you cannot prove A then you must assume not A is true has a great deal of problems.

The first of these problems is when not A cannot be proven either. If there is no proof for not A, must we also assume that A is true? Both of the statements cannot be true at the same time. I have no reason to believe of the existence of a mind of a noder who simply copies and pastes the works of others, must I assume that it does not exist?

This approach leads to absurdity with numerous other classical problems in philosophy. Of these, my favorite is the 'red green inversion'.

Can it be determined if what I perceive as 'red' another individual will perceive as 'green', and what he perceives as 'red', I perceive as 'green'. Now, we both call the colors the same, its just that the mind-state that is caused from the perception is reversed.
Many philosophers puzzle at this for awhile, some call it a silly problem because you can't ever figure out if that is the case or not (some jokingly argue that this is the reason for bad fashion sense). Now, applying the rule above, if we cannot determine if if we perceive colors the same, we must assume that we perceive them differently - without any evidence backing it.

When there is no proof for something, the correct response is not to assume that something is not the case, but rather suspend judgment and continue to seek. If there is no proof for or against it, then feel free to believe whatever you wish.

One of the interesting parts of philosophy (again, depending on your bend) is the argument that there is no interesting philosophical conclusion that can be proven beyond the possibility of doubt. That there has been no argument for the existence (or non-existence) of God does not weaken (or strengthen) the case for God's existence. Simply, this places the question of God's existence in the same set of questions as the nature of an external, mind-independent world or the question of how we know other individuals have minds at all.

Many philosophers have taken up the challenge of a proof of God's existence or non-existence. Some have even tried to prove the impossibility of the proof for or against the existence of God (see Critique of Pure Reason). However, there are reasonable proofs on both sides of the argument. The question is not one of "overwhelming proof" but rather "consistent with the evidence and of explanatory value".

Throughout the history of science, observations where made about cosmology going from a flat earth through a geocentric universe to Kepler and elliptical orbits about the sun. None of these beliefs were unreasonable at the time of their conception and provided explanatory value for the state of the world. Ptolemy's system worked for centuries and provided a great deal of explanatory value.

Today, we have quantum theory and super strings that we have no proof of. The stuff of quarks has only been theorized - there is nothing to say that that is the actual way it works. And yet, we believe in them because they are consistent with the evidence and provide explanatory value for us.

If it is reasonable for us to believe in scientific theories that have no proof but rather observations consistent with the theory, it is just as reasonable for people to believe or disbelieve in the existence of God or gods.

One of the problems that people appear to be having here, is the question of belief. Belief is one of the requisites for knowledge, along with truth and justification. However, lacking proof of the truth we are left with the issue of 'reasonable belief'.

What does it take to have a 'reasonable' belief. Here, the requirements are even less strenuous than the justification for knowledge.

If you have an argument that convinces you, or as I've said above an explanation that addresses the evidence, then this is a reasonable belief. To a child, who is told by an authority that Santa Claus exists, the belief in Santa is a reasonable one.

  • Santa is written on the from on presents.
  • Santa gets mail sent to him (it doesn't come back)
  • Santa listens to your wishes when sitting on your lap, and you get those presents
  • Santa is part of the weather report on Christmas Eve
  • etc...
Does not the belief in existence of Santa Claus fit the requirements for 'consistent with the evidence and of explanatory value'?

Similarly, the belief in God is reasonable. There are a vast many people who hold that His existence is consistent with that of the evidence of the way the world works and provides explanation as to the way the world will continue to work.

In both of these cases, the issue of 'truth' does not come into play. Most of us would actually be worried about a young child skeptical to the point of not believing in Santa when given such a preponderance of evidence (especially if they haven't been told that he doesn't exist). Likewise, religious authorities are often concerned with people who do not believe in God in what they consider a similar set of evidence.

We are not dealing with a criminal court here. The criteria for reasonable belief is not that of "beyond all reasonable doubt" - that is knowledge. Only a very few deluded individuals will make claims to having knowledge of the existence or non-existence of God. Even in a jury, jurors often have the responsibility of saying "Although I believe that A, it has not been proven so I must say not A".

Reason to believe is not proof of knowledge.