I'm afraid, Igor, that your arguement is simply a GotG (god of the gaps) arguement. You are suggesting that athiests can't accept the big bang because it is not known what kicked it off and what came before.
The big bang happened, there is too much evidence for it. Where it came from isn't. Bringing the law of thermodynamics into it shows that you, as Jar suggests, do not understand the science involved at all. You should probably avoid topics that you know nothing at all about until you are ready to ask questions and understand (to the degree that those of us who aren't physicists can) the answers.
If there is something unknown it is hardly an athiest that would think that a god has to be put forward as the explanation. They would know from history that this has been a bad idea in the past.