The second law of thermodynamics states that matter in a closed system such as the universe will move toward a more disorganized state assuming outside forces are not acting on the universe. If we assume that the universe is a closed system and that there is no god (outside force), then we should see disorder growing around us. But the net level of disorder in the universe does not appear to be increasing. It appears that the opposite is happening. With the exception of rare supernova, everywhere in space we see evidence of highly organized planetary and biological systems forming.
However, the second law of thermodynamics also states that disorder (entropy) may decrease in a closed system by chance alone. This means that star and planetary systems may organize without influence from an outside force (i.e., god). Atheists kind of like this part because it suggests that complex organization can occur without divine intervention. Now I argue that the chances of spontaneous complex organization (low entropy) occuring by chance are very slim, but in all fairness to atheists, as Jim Carey's character on Dumb and Dumber reminds us, even with extremely small odds, there's still a chance.