Atheism is inevitable secular progress. Its existence is a thing society needs. However, too many atheists act like missionaries -- evangelizing at strangers who just want to live their lives, feeling and acting smugly superior because of their (non)beliefs, and insisting that a greater good justifies their asshole behavior. I would know; I'm an atheist myself, so I talk to them all the goddamn time. So it's especially frustrating when my more devoutly atheist friends don't realize that not only do a lot of their arguments fall on deaf ears, but also how in the long run, they hurt their own cause way more than they help. Here are a few examples.
#5. "There's No Scientific Proof"
There's absolutely no scientific proof that points to the existence of gods. We've scanned nonillions of cubic light years and delved into the smallest subatomic structures without finding a single speck of divinity. It seems like the best possible argument to an atheist, but that's just because they lack faith. Or rather, they lack a basic understanding of what the word "faith" means. Here, allow me:
You could've done this yourself years ago.
See the last bit in the second definition? Or just the first definition in its entirety? Having faith means you don't need proof. Scientific argument can't convince anyone subscribing to a science-less universe. You're basically telling them that they shouldn't believe because ... they shouldn't believe.
But a scientific argument doesn't need to convince the believer that there is no god. It just needs to keep their religious beliefs the hell away from the things that allow a modern society to advance. If they were trying to build a functioning airplane supported by the power of faith, you could do the physics calculations using Newton's laws to show them why they should trust science instead. If they still refuse to change their stance, the ensuing plane crash will do the job.
Whatever the case, at some point, science is winning that argument.
However, if they're using religion to support themselves emotionally, well, Isaac Newton would have supported that idea. He was extremely religious. He thought the beautiful laws of reality were the masterwork of a divine creator. Science and religion work fine together as long as they stick to their own roles. It's only when religious ideals intrude on scientific fields like particle physics or basic medical care that things get scary.
#4. Logical Paradoxes
People aren't Star Trek computers. You can't explode them by feeding them a logical contradiction. That doesn't stop atheists from deploying cute little jabs like "Who created god?" and then using the lack of an acceptable answer as proof that they've won the argument. The main problem with this (beyond it not changing any minds whatsoever) is that it's impossible to make a point like this without seeming like your real motivation is to make the other person feel stupid, and that's a terrible way to conduct an argument.
Most people have their faith instilled by their parents or culture before they've even learned to think for themselves. It's just something they're told and believe. Trying to make them feel silly for it with a series of catchy one-liners will come off as a personal attack, whether you mean it to or not. Besides, if someone is Vulcan enough to change their entire mindset and belief system according to one line of logic, they're not going to believe in a god anyway.
But I bet they have the exact same feelings about a version of Linux
The most common such question is "Why does a god let bad things happen?" It's an excellent question, but one the everyday believer on the street has no need to answer, or have answered for them. Again, that's what having faith means. "God works in mysterious ways" and all that. You aren't going to change any minds by reminding religious types that serial killers exist and tsunamis happen.
Asking why anyone would worship a god that lets bad things happen is like asking someone in an abusive relationship why they don't just leave. Even if you get an answer, it's not going to make sense to you, or further the discussion in any sort of meaningful way. Even if you think you're bringing it up for their own good, shouting "You're an idiot for staying!" isn't how you help. Just let them know that options to leave are available.
#3. "The Bible/Torah/Quran/Tripitaka/Whatever Is Full of Screwed-Up Stuff!"
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Of course they are. Anything written millennia ago is going to be full of horrifyingly outdated instructions, the same way any Bugs Bunny cartoon from the early '40s stands a great chance of being super-duper racist. It was a different time, you know? That's why it's so important that religion and law be kept separate. Laws govern current society. Ancient texts literally record a more primitive society. A government with a mandated religion is about as backwards as a government can get while retaining the ability to write things down.
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Writing things down means never having to think about them again, no matter how much things change
But most quietly religious people don't adhere to religious texts in a literal kind of way. There's a word for people who do, and that word is "fundamentalist." If the person you're arguing with falls into that category, then chances are they've spent years reading those texts and internalizing their literal meaning. Nothing you say is going to cut through that in the slightest. That'll take years of deprogramming.
If someone starts screaming that people shouldn't be allowed to marry because of a millennia-old fanfiction, then yes, scream back. Or even better, go make sure their screaming has no effect on anyone else. But unless you see a Christian trying to sell their daughter as a slave or otherwise use an old book as some sort of shield against modern society, you don't get to deploy Exodus 21:7 against them.