Atheists, I am not here to talk about Christianity or any other religion.
OK – I won’t go through the ‘you are an atheist about the 5,000 other gods’ routine.
I want to know exactly why you do not believe there is a Supreme Being.
Parsimony. Occam’s razor. The need for extraordinary claims to be supported by extraordinary evidence, and the total lack of any evidence, extraordinary or otherwise, in this instance.
For the sake of argument let us say that evolution is real
OK. That’s a fair interpretation of the mountains of evidence, and complete lack of cogent competing theory.
and the big bang began everything.
Not at all OK. Physicists do not know, but are trying very hard to find out, and the kind of possibilities they are looking at make both ‘began’ and ‘everything’ non-trivial items to define – metaverses, branes, M-theory, time-loops and so on. We don’t even know the question, let alone the answer!
What created the means necessary for the Big Bang to take place?
See above – we don’t know (and neither do you), but we are going to try very hard to find out! Any other answer is an Appeal to Ignorance, ie a logical fallacy.
What was at hand to spur mutations and changes to cause evolution to take place?
You don’t understand evolution. There is no requirement for a teleological ‘driver’ – that’s why it’s such an elegant solution.
Why do you look at a car and know that there was something involved in the creation of it and that it did not simply fall together by chance
Because, based on what I know of cars, and of reality, that is the most likely explanation.
and at the same time think that trees are a mistake?
You are confusing complexity and design. Complexity occurs from simplicity quite happily in nature. Even if you pare things down to a simple computer simulation, it still happens – it’s that ingrained into our universe.
I do not think trees are a mistake. That would be anthropomorphic and not even mean anything.
I like trees!
Why is it so hard to comprehend that the universe was made by God?
It’s not hard. It’s very easy - that’s why primitive societies did exactly that. It’s not necessarily true, but compared to postgraduate quantum physics and cosmology, it is very easy.
Even if the universe is a mistake
I do not think the universe is a mistake. That would be anthropomorphic and not even mean anything.
I like the universe! Although it has room for a few improvements…
1 in 1, it would appear! Again, we just don’t know.
then what was it that made this possible?
What made it impossible? Possible / likely / unlikely compared with what? Again, we don’t know enough to define, let alone answer these questions, yet.
We’re agreed on something!
and have the ability to disprove there is a God
I cannot disprove any negative. The onus is on you to prove the positive.
yet you have the audacity to think there is no God that created you and gave you the ability to disprove Him. There is no God who gave you this ability?
I am audacious because my non-belief that I was created by a non-existent deity negates my non-existent ability to disprove a non-existent deity? You’ve lost me now!
Remember: YOU ARE ALIVE! There had to be something that made that possible.
Well, by definition, since I am indeed alive (thanks for the update!

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I think you are confusing ‘how?’, which is what scientists are asking, and ‘why?’, a human-specific attribution of ‘meaning’ and ‘purpose’ that we can in no way apply to nonhuman affairs.
Again, if this was random chance, what was it that caused this?
See above.
HOW can you believe there is no God?
In the same way as you can believe there are no unicorns. Zero evidence.
Have a nice day!