When wanting to proselytise a belief, it's important to understand how to effectively communicate it. This means understanding where the other side is coming from. In my time on atheists forums and blogs, I've come across many Christians who try to proselytise their beliefs and finding one who can actually communicate with an atheist about beliefs is almost non-existent. The ones who believe in God simply don't know what it's like not to, and as such their arguments reflect this basic ignorance of the position they are arguing against. Here are 10 of the most common ineffective arguments against atheism I've come across, in the hope that theists will take it on board for future communication.
Quoting the bible
Certain passages may be emotionally significant to you, but there's no reason to suggest that someone not of the faith will be swayed by them. Consider that a Muslim was trying to convert you to Islam. And his survival strategy was to quote the more eloquent and profound poetry out of the Koran. Would that sway you to become a Muslim? If not, what makes you think it would be any different to an atheist hearing passages from the bible? There may be some wonderful passages in there, and there may have been some profound things said by Jesus and other figures. But it's not going to sway those who don't have that emotional connection between the bible and a belief in God.
The threat of hell
It may be that the threat of eternal torture is a very real threat, and that you need to save as many people from that threat as possible. It's a sign of a good person to want to help as many people as possible. But it's important to remember that it's only a threat if someone believes it's a threat. Consider a Buddhist warning you on The Cycle of Samsara, and that if you don't follow The Noble Eightfold Path that you'll suffer in the next life. Would you seek to end the cycle of suffering by following Buddhism? If the threat is not sound real, people will ignore it. It goes for anything, if one doesn't know what a gun does they are hardly going to be scared if one is pulled on them.
This might seem like a good strategy, but like the threat of hell there needs to be a reason to believe in the reward on offer. If someone were to offer you $50 billion dollars in exchange for cutting off your arms, would you go and cut your arms off without any assurances that the act would result in the said compensation? Rather one would hopefully get assurances that the money is there and that there are measures to ensure the transaction goes ahead. It may be that simply believing doesn't have the cost that swapping your arms for monetary gain has, but the risk of believing is still there. What if we believe in the wrong god and that god is petty and spiteful? What if eternity was achieved through works rather than faith? In the absence of knowing, the wager is a 1 in infinity stab in the dark.
Jesus may have died for our sins, but it's hardly a selling point to those who don't believe in the concept of sin in the first place. To those who find the concept of original sin absurd to begin with, there's no need for any martyrdom in order to atone for it. Again this may be emotionally significant to you, and if The Passion Of The Christ is any indicator then it would seem that it's a common belief. But without the need for atonement, without the belief that atonement can be achieved vicariously through the suffrage of Jesus, arguing that Jesus died for the benefit of mankind is no better than arguing that one should be a Hindu on account of Krishna's similar plight.
Calling someone immoral off the bat is never going to win them over, nor is holding them accountable to your moral standard. Would a Jew calling you immoral for eating shellfish and working on the Sabbath sway you to forsake Christianity and become Jewish? Or a Muslim telling you that you're immoral because you enable a society where women can wear miniskirts persuade you to join their faith? The same goes for atheists, it matters not whether you think God hates homosexuals or that abortion is a sin. When one doesn't believe in God, appealing to God as a basis for morality is not going to persuade anyone.
Hitler / Stalin was an atheist
Throughout history there are bound to be humans beings of all walks of life who have taken part in atrocities. But it's guilt by association to argue that because Stalin was an atheist that atheism is bad. Hitler was a professed Roman Catholic, but does it matter one bit about the question of God whether Hitler believed or not? Would Christianity automatically become evil if it turned out that Hitler (as his writings and speeches suggests) was indeed a Christian? Does the witch-burning throughout Medieval Europe or the numerous inquisitions that led to the slaughter of heretics invalidate Christianity? And does this at all have any bearing on whether there is a God or not? It's appealing to social consequences (which are largely unfounded) in order to weigh in on a philosophical position.
The former atheist
Antony Flew may have now gone deist and CS Lewis may have been an atheist in his teens before becoming a strong apologist for Christianity, but what does it have to do with whether God exists? Consider the opposite situation where Christians have rejected a belief in God and turned atheist, does their rejection of Christianity prove that God doesn't exist? When preachers like Dan Barker or John W. Loftus left the flock and are now prominent atheists, did that hurt the arguments for God? If not, then what does Antony Flew's conversion to deism have to do with the arguments for God? By focusing on the authority of the figure as opposed to the arguments used to justify such a conversion, it will not resonate with atheists at all to bring it up. Atheism is not about what authorities say.
Arguing against atheist "authorities"
It must be recognised that atheism is a non-belief in any god, nothing more and nothing less. As such, there are no atheist doctrines or dogma, no prophets or proverbs. Think of what it means to not believe in astrology, and how not believing that the destiny of our lives are written in the stars means. A non-astrologer does not regard James Randi or Carl Sagan as prophets, much as an atheist does not regard Richard Dawkins or David Hume as such. Even if a particular argument that Dawkins uses is bad, it doesn't make the case for God any more convincing. Likewise, even if an argument Dawkins uses is valid it does not mean that all atheists have that as part of their belief in God.
Ignorance is no crime, with the sheer amount of information out there it would be impossible to expect one to know everything. But the problem is confusing that personal ignorance for a wider ignorance on the matter. Science has come on in leaps and bounds, and if you are reading this page then you are at least somewhat aware of the power of the scientific method. You may not personally know how life came about, or how planets and stars form, but that doesn't mean that others share this ignorance. Would you listen to someone who tried to tell you that God was really a giant tyrannosaurus called Frank, and that Christianity was founded when a dinosaur stumbled into a space-time continuum and inadvertently created the big bang? Especially given the links between atheism and methodological naturalism, arguing on scientific grounds will require an understanding of the current scientific knowledge base.
The universe is very intricate and complex, and as a species we are incredibly complex beings. But there is a process by which complex life can emerge called evolution. As such the appearance of design can be fulfilled by natural processes. It's neither accident nor design that when you let go of an object that it falls to the ground, it's simply the gravitational attraction of the earth. The dichotomy between chance and design is a false one, so challenging about how it all came about by accident misrepresents how a scientifically-minded atheist views our origins.