Citing the Bible
If you come into an atheist forum citing scriptures at everone then let me inform you my dear theists that you are wasting your time for several reasons in particular. First on the list is that all atheists don't accept the Bible as the inspired word of God as you do. It is not inerrant to us and is actually full of things we would deem self-contradictory, immoral, and disgraceful. If all you can do is cite scripture without having any background knowledge as to how those scriptures came to us or how they were composed, compiled, and edited then I am pretty sure that you have little to contribute to any discussion refuting your faith.
It's not the word of God to us!
Threats of any kind tend to be frowned upon and it makes you look like an ass. When it comes to divine threats speaking for myself I tend to become combative. Not because I find your threats credible, but because I find your threats to be a sign of your arrogance. Secondly, I find your threats to be empty just like your beliefs. Think of it this way: atheists don't believe in gods so threats about your gods judgement on us is not even a threat but rather a sign of your delusional state. Threats about hell and what awaits us evil nonbelievers in the afterlife are even less effective since most atheists I know don't believe in an afterlife, although there may be some exceptions out there. So threatening us with God's wrath, hell, or the great white throne judgement is like threatening an adult with not getting anything for Christmas but a rock of coal from Santa.
Hell is about as real as Oz it doesn't exist!
Throwing people out
Nothing gets under my skin more than a Christian who comes into an atheist forum and then commences to verbally demand that you leave the forum if you have nothing nice to say about Jesus! This is just fucking rude! For the record and I can't speak for other atheists I personally don't go around trolling Christian discussion forums to insult your beliefs or annoy believers. I think that that is childish and not worth my time. But if you come into an atheist forum preaching and condemning everyone else to hell then get offended when you are attacked I would have to say that you brought it on yourself. For the most part we atheist love to have rational discussions about religious beliefs but if you can't be rational then save yourself the trip.
If you are easily offended by images of Jesus in compromising positions or reading blasphemous material and seeing vulgar jokes about your religion then definitely an atheist discussion forum is not for you.
Not cool! Especially of you're the guest in the forum.
Nothing is more annoying than circular reasoning! What is circular reasoning?
Circular reasoning: A type of reasoning in which the proposition is supported by the premises, which is supported by the proposition, creating a circle in reasoning where no useful information is being shared. This fallacy is often quite humorous.
Here is a perfect example of circular reasoning:
Proposition: The Bible is the word of God
Premise: Because I believe it and it says so right there in the Bible
Keep in mind that to us atheists the Bible, Torah, or Koran is not considered evidence of anything. They are considered for the most part to be the works of men of an ancient and very superstitious time.
Circular reasoning is not reasoning at all!
Prejudging or making blanket statements
This is a cardinal no no when it comes to any discussion forum. Assuming that atheists don't know what the Bible says, are immoral, don't understand the scriptures, are angry at God, don't believe in him so that they don't have to be accountable for their actions, are in denial because of their love of sin, etc. If you do any of these chances are you will bring upon yourself the wrath of an entire atheist community and will be bombarded by equally or greater offensive blasphemy meant specifically to piss you off!
Assuming what we believe
Don't ever assume that you know what atheists believe! The only thing any two atheists can agree upon with certainty is that they don't believe in gods. That is all that atheism is we live our lives as if gods did not exist because we believe that there is insufficient evidence to substantiate the existence of gods. Our personal views about religion in general, politics, the world, science, nature, etc. differ greatly from person to person.
Atheists don't believe in gods or any other supernatural entities described in your texts. This includes angels, demons, saints, Jesus as the son of God, divine interventions known as miracles, end time prophecies, heaven, hell, purgatory, the concept of sin as an offense against God,the fall of man, Jesus rising from the dead, etc.
Citing Christian sources
Just like we don't accept the Bible as the word of God we don't accept most theistic web sources that misrepresent science or atheist views. Most of these sites for the most part are either misinformed or purposely deceitful. For example when arguing against the Big Bang or evolutionary theory please save yourself the embarrassment of citing banana man Ray Comfort or others like him or creationists websites like answers in Genesis. If you want to learn how not to learn about science this is definitely the website to learn how to make a fool of yourself while discussing these issues with atheists.
Atheism is a religion
Warning if you make this statement prepare to reap the whirlwind that will follow! Not because it's true as you might assume but because it is one of the most annoying things you can say. It demonstrates to us that you don't even know what atheism is. Atheism is basically the belief that there are no gods that's it! We don't worship anyone or anything, we don't pray or say mass, we don't have unholy texts, we don't all agree on other issues not related to the basic idea of atheism, science is not our god and Richard Dawkins is not its prophet. Our beliefs are based on reason and logical deduction.
Don't use Pascal's wager!
In its most basic form Pascal's wager states that you should live as if God exists because if he does and you don't believe the consequences of unbelief are far greater than if you believed. Basically, it is more logical to live as if God existed in the long run than to believe the opposite and have to face eternal consequences in the afterlife. There are many ways to debunk this position and I think that you will find a good refutation to it here.
God of the gaps theories are despised
Another thing theists should not do is try to debunk atheists arguments about origins with god of the gaps theories. This is what is meant when theists specifically attack gaps in our current scientific knowledge. A common point of contention is the one of origins. This refer to the origins of the universe or the origins of life theories. Christians love to throw out the old "if God does not exist then where do you think we came from or how we came to exist?" question. But in all truth I tend to reply that neither I know for sure and they are in the same position as I am since their only source for their beliefs is a book supposedly inspired by divine revelation. Unfortunately dear theist we don't accept divine revelation as a source of knowledge for anything. Therefore in all truth you are as ignorant to the origins of the universe or life itself as the rest of us on this planet are.
I hope that this short post will help you out when you decide to enter atheist groups or discussion forums. I'm sure some of my readers could add more to this list but I think that these are pretty much the greater offenses committed by most theists.