In the course of life's wanderings I have, as an atheist, been involved in discussions with theists. Here are a few of the "arguments" I have heard, and why I dismiss them.
Some people argue that "theists can't prove god and atheists can't disprove god, so their respective views are equally justified." I have a short satirical response that reveals the idiocy of this thinking: "fairyists can't prove fairies and afairyists can't disprove fairies, so their respective views are equally justified."
Some people argue "what's the harm in believing something that might be false, when it's done so much good?" So much good? What's the harm? Are you kidding me?! Yes, please let's forget about the crusades, the inquisition, the conquistadors, the witch hunts, the opposition to health care research and measures, the attempts to utterly destroy science by redefining it to include such idiocies as "intelligent design" and astrology, the murdering of doctors on "moral principle", the child abuse (e.g. "hell houses", priests, & more), the constant attempts at religious legislation, and much, much more. It's like defending Hitler because of all the good things he did. (Yes, Hitler did good things. But, before you go running off claiming I'm defending him, let me clearly state: I am not. What he did was horrible. But, that doesn't change the fact that, had he died a few years earlier, he'd have gone down in history as one of Germany's greatest leaders. And, ironically, my comparison is even closer to the mark than many realize, since Hitler received his anti-Semitic ideas directly from Christianity, via the Catholic church. (Look it up!) So, religion could even be indirectly blamed for the Holocaust.)
Many will protest my examples here, but unjustifiably so. Every one of these things is/was directly driven specifically by religion. Furthermore, I haven't even begun to scratch the surface. This is but a selection from an enormously larger pool just from Christianity. I shouldn't even need to get into all the various religious wars fought (northern Ireland, eastern Europe, the Middle East, various parts of Asia, etc.), both between Christian sects and between completely different religions... the jihads and fatwahs, the Jehovah-ordered genocides, human sacrifices, and slavery in the Bible, etc. Think for a moment about conflicts around the world, and the automatic "lines" that get drawn between groups---how those groups get determined, and what reasons people use to "justify" the conflicts (e.g. George W. Bush claiming God told him to invade Iraq). Please, actually think about this Richard Dawkins quote, instead of just "gut reacting" to the speaker and content: "Religion is the most inflammatory enemy-labelling device in history."
Some people argue that "religion improves people's behavior" (as individuals). Ignoring the fact that this assertion bears no relevance to the truth of religions, let's consider if the assertion itself is true, anyway. The short answer is a resounding "no". To cite just a couple supporting facts... The 2005 publication by Gregory S. Paul, in the "Journal of Religion & Society", noted a fairly strong correlation between increased secularism and improved societal health (in multiple indices) of countries. (The paper is entitled "Cross-National Correlations of Quantifiable Societal Health with Popular Religiosity and Secularism in the Prosperous Democracies".) In other words, the more religious a country is, the more socially dysfunctional it tends to be. (Why this is true is unknown, but it is true, nonetheless.) The Christian sociologist George Barna's published divorce rates: "born again Christians" 27%, "Mainstream Protestants" 24%, "Atheists/Agnostics" 21%. Other data imply that atheists/agnostics are involved in less crime than theists, too. As Steven Weinberg said, "Religion is an insult to human dignity. With or without it you would have good people doing good things and evil people doing evil things. But for good people to do evil things, that takes religion." And as Voltaire said, "If we believe absurdities, we shall commit atrocities."
Some people argue (usually as a last resort) that "the atheistic view of life is empty and meaningless" and that "people can't live without purpose" and that "religion makes life all warm and fuzzy." Even if I agreed with these assertions (which I do not; see my "Mainstream Religion Versus Secular Humanism" blog), they still bear no relevance whatsoever to the truth of the issue! In other words, they're not real arguments (just like the preceding ones).
Even the attempts at rigorous arguments by Thomas Aquinas (who is unjustifiably revered in Christianity) and William Paley (whose argument "intelligent design" basically restates, 200 years after the fact) are ultimately vacuous. These arguments include such things as "first cause" or "prime mover" or "design/designer". If you state "everything has a cause" as an axiom, then you cannot immediately follow it up with an inference that there must therefore be a "first cause" (or "prime mover"). The very idea is absurd in the extreme because, by so doing, you have negated your axiom. The argument that "the appearance of design implies a designer" is also a non-solution because the real problem lies in explaining the improbability of a thing. And, you can't solve the problem by positing something even more improbable, more complex---which a designer presumably would be. More succinctly: if god designed the universe, then who designed god? And, who designed god's god? And... The argument is either valid, or it is not. You can't simply pick and choose to what it applies. And, it solves nothing. (In fact, it only makes the situation infinitely worse.)
Ultimately, every in-favor-of-god "argument" I have ever heard has turned-out to be entirely devoid of relevance and/or meaning.
In the end, religion all sounds like this to atheists: http://jhuger.com/kisshank.php