Couple of things here for the OP to note. This argument (pretending it's valid) doesn't cause much of a blow to atheism because
atheism doesn't require the need for any justification. People can try to justify saying a god exit, or a god does not exist, but some simply think that there is a god, or that there isn't. Not everyone looks for justification in their beliefs. Also another flaw with your argument is that theism shares every flaw as the big bang. Something can't come from nothing. That in essence is your argument against the Big Bang. Yet your beloved deity came from nothing, but I guess that's perfectly okay because he's magical and can do whatever he wants right(sarcasm intended)? Anyway your argument against atheism is hypocritical /nuff said.