A world of majority atheists would be bleak because Atheism, imo, is a very depressing and bleak worldview. Everything is an accident, we're all here for no reason at all. It makes life utterly worthless and meaningless.
You know most atheists don't think "everything is an accident", right? That's a gross oversimplification. Random chance may have been involved, but if our universe truly came from naturalistic forces alone then given enough time something like our universe coming into being may well have been inevitable - give infinite monkeys infinite typewriters and infinite time and you'll eventually get Shakespeare.
And as far as meaning, we can assign our own meaning to our existence. I mean something to myself, as to the people who are important to me. I don't need some outside person telling me what my meaning is. I can decide that on my own.
Also, if you want to talk about bleak worldviews, how about the one where all of humanity is wretched, evil, and unworthy and need to be saved from eternal hellfire because we fail to live up to a standard of perfection that we could never possibly reach but are held to anyways? It's called Christianity, and frankly it's pretty damn bleak if you ask me.
Atheism doesn't necessarily lead to rationality, that's true, but religion inherently requires believing something fantastical without evidence, which is pretty irrational when it comes down to it. Between the two, atheism is more likely to lead down the rational path IMO.
First off, moral relativism is a fact - people have different morals. You can see this in your own community as well as across the world. There is no one set of moral rules people follow.
Second, nobody said everything is nothing. Existence is existence. In this world behaving badly can have very bad consequences. I don't need an invisible man in the sky to tell me that murdering someone is going to potentially piss a lot of people off and make them come after me. We are social animals by nature, and some of our morality is hardwired. If you've got a spare hour you should listen to
this lecture on the subject.
Third, from the perspective of someone who doesn't believe in the Bible I find much of it's contents that are espoused as just and moral to be morally repugnant. That doesn't mean I don't think there's anything good there, but there's a lot of horrible stuff in there such as justifying the mass murder of women and children, slavery, stoning people for various reasons I don't find justified at all, the treatment of women as a kind of property, etc.
You do realize that most people who call themselves agnostics are actually atheists, right? All that is required to be an atheist is to not have a belief in any gods. Atheism is about what you believe, agnosticism is about what you know. I'm an agnostic atheist - I don't believe in a god, but I don't claim to have knowledge about whether such a thing does or does not exist, at least in the general sense.
Maybe the more vocal ones are, but did you ever think that maybe you just don't notice the non-vocal ones because they aren't vocal about it?
Also, there is no atheist dogma, so I don't know how they could be dogmatic about it.
Here's the thing - you can't blame the horrors of communism on atheism because atheism wasn't the driving factor behind that. Atheism only requires not believing in a god, and there's nothing directly linking that belief to deciding to create a tyrannical regime that kills and oppresses people. You have to add additional beliefs to that line of thought to get there.
On the other hand I can blame Christianity for many of the horrors done in the name of Christianity such as witch hunts. As I stated, belief informs action. Part of the Christian Bible states quite clearly that witches should be put to death. Therefore if you are a Christian and you believe someone is a witch then your Christian beliefs dictate that you must put that witch to death. It's part and parcel of the belief set - to say it isn't when it's clearly written in your most important holy text is intellectual dishonesty.
Also, you can't blame it on insanity - these actions were not performed by insane individuals. If they were those individuals wouldn't have made such a large mark on history. These actions were done by large groups of normal people with perfectly functioning minds.
Yes, they should be abandoned too.
I can't think of one religious good that actually requires religion to do it. For instance I don't have to be a Christian to give to charity.
Again, many atheists do not actually say there is absolute certainty that there are no gods. Atheism is only not believing in any gods. Not believing and not acknowledging possibilities are entirely different things.
Even those who claim absolute certainty are probably less arrogant that fundamentalists are. The reason I say that is simply that they are rejecting a fantastical claim. I mean seriously, would you say someone was arrogant for saying that they were absolutely, 100% certain that leprechauns didn't exist?