Burden vests with atheist to prove that the stance of taking the opposite proposition as true is valid.
As Alex Michalos says in his
Principles of Logic. Englewood Cliffs: Prentice-Hall. p. 370.
"usually one who makes an assertion must assume the responsibility of defending it. If this responsibility or burden of proof is shifted to a critic, the fallacy of appealing to ignorance is committed."
If I find evidences in favour of your proposition inadequate, I cannot claim that the converse proposition automatically stands proven.
And the atheist doesn't say his proposition automatically stands proven. All the atheists I know of don't care whether you believe them or not. All they are saying is, "you haven't substantiated your claim that god exists." You presume too much here, atanu.
Should a sailor assert that giant gem-encrusted sea serpents exist, as a doubter faced with a lack of convincing evidence I feel confident in saying that they don't exist. And I feel no burden whatsoever to prove my lack of belief.
The atheist has no interest in what you think of his disbelief, which is unlike those who assert the existence of god. Theists make such an assertion in expectation/hope that they will be believed--why else bother making it? A burden is only created when an assertion is expected to be taken as true. Atheists have no such expectation. Don't want to believe me that god doesn't exist? Fine. No skin off my nose.
Absence of evidence cannot constitute evidence of absence.
The theist position is an assertion of fact: there is a god. Not, "
I think there is a god," or "
I believe there is a god," or even "
I have faith there is a god," but "
there is a god." Were it not for this assertion there would be no atheistic position. Atheism only exists because theists have declared the existence of a god. So the burden of proof falls only on the theist.
Without the assertion that giant gem-encrusted sea serpents exist I have no reason to bother asserting they don't. People don't go around saying that purple dotted dogs don't exist in nature, or that there isn't a real Superman, because such claims are unreasonable if not outright stupid. However, they may very well do so if some knucklehead insists that these oddities do exist.