Raza, many atheists do question what the real truth is, etc. I think that many haved looked critically at religion, especially as it has changed over time, and have come to the conclusion that it can't have been true because it has changed so much, or they have come to the conclusion that religion has negative effects generally.
Atheism isn't about the Big Bang, in fact it has almost nothing to do with it. The Big Bang is just a theory without the negatives of religion, it doesn't have to be justified against religion because religious theories are disqualified.
So I think the position of most atheists is that religion just doesn't make sense. I'll use a Christian example. Suppose God is all powerful, he can do whatever he wants. Then why would he make this world? He could counter any need or urge he had. Why should God care if some ant on a ball of rock prays or not?
I think atheists also see that most religious people use religion in strange ways. For instance an earthquake happens and many die, but one of the survivors says "thank God I'm alive". It seems rather callous to suggest that God would let you live but others die. Why shouldn't God have prevented all deaths?
Also there is the matter that religion allows people to be manipulated. People will do what they can to evidence their belief, so like a boyfriend who says "if you love me, you'll have sex with me" they can be manipulated by fundamentalists. Suicide bombers profess to be following Islam. Whether or not it is the real Islam, there is still the matter that they think it is, and I think that says something about religion, that it can be used in that way.
So faced with these reasons, I think atheists reject religion and religious theories, not because they are demonstratively untrue but because the effects of that belief appear to be very negative, to the degree that religious belief seems irresponsible.
Many atheists don't try to persecute others but rather want to be allowed their autonomy. They would rather have the state follow secular values so as not to favour any religion. Science has no opinion on religious matters, it proposes theories that help explain the world but can't disprove religion. The Big Bang is a theory that science proposes but it's not something to believe in, it's something to accept as useful, whatever the truth may be.
So perhaps there is a balance between two things, the belief in the existence of some God and the recognition of the effects of religion. For most atheists, I think the effects of religion weigh heavily enough to favour a secular society which only means that one may believe what one wants to believe but should not force that choice on others, that one should not prejudice the choice of others. Religious freedoms are respected in a secular society, but they are not allowed to prejudice other people's freedoms.
Of course, if one firmly believes in some religion, they are going to want to evidence their belief because the proof of the pudding is in the eating, and what type of Christian or Muslim or whatever would you be if you didn't act accordingly? If those religions tell that one should spread the word or whatever, how could one call oneself Christian or Muslim if they didn't do that?
So in this sense, it is probably a bit fantastic to expect that religious people would come to respect secular freedoms. We could only expect them to do that if it was consonant with their religious beliefs. For this reason, some statists would rather have the state take the place of religion, because to them allowing religious freedoms will inevitable have the effect of having people speak out against secular values. If secular values can't be had while religion flourishes, they then seek to institute a new religion.
So I don't think one should think of Atheism as another religion; it is something altogether different. Atheists choose secular freedoms over religious monopoly. Atheists differ in their political aims of course, but in general I think it is a reaction to the nature of religious belief, that it might preclude respect for secular values.