Why does it matter so much to atheists that God doesn't exist?

IT does not matter, mainly because it does not exist.

Notice that I said their rules, not any gods rules.

These are man made rules - made thousands of years ago for the use of the humans who lived thousands of years ago.

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I am speaking to the US Bible thumpers here, of course ~ many of whom are flat-earthers who deny evolution and see scientists as evil people who are constantly trying, and succeeding, in disproving their beliefs one by one.

Because these are not any gods rules we are talking about.

The people of the modern world have evolved past all of that nonsense.

Why did the massacre of al-Dawayima Palestinian residents not have the same reverberations as the Deir Yasin massacre?

I do not personally care if you believe in a god or not.

And how do you deny hundreds of years of advancement in all fields of science?

Those rules that you are trying to reinstate are rules made by men who didn't know anything about the earth, nor anything about all of the other inhabitants of it. How can you not see that?

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My problem is that people who try to shove their rules down everyone's throats - including by physical force - DO EXIST .

And then use and ENJOY these fantastic scientific advancements every single day of your life?

THE BIBLE IS THE PROBLEM!

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A belief in a god is not the problem.