I argue because I care about people, because it saddens me to see people frittering away a good portion of their very short time on earth in pursuit of pointless things, like worshipping and praying to a malicious supernatural entity, e.g. the Abrahamic God. Think of how many things you could otherwise be using that time for, things that would actually be helping to make this a better world to live in.
I argue because this is one of my moral values, to place the welfare of human beings in the here and now above any and all unfounded and speculative beliefs of a better life(?) to come once we are dead. How does it make any sense to say that in order to enjoy a better life than we have now, death is mandatory? To hold that view is to say we'd all be better off dead, (presuming we've all first converted to Christianity, in your view). If that isn't a notion worth arguing against, what is?
I argue because willful ignorance is a cancer of the mind. To go on proclaiming belief in some principle that has been thorougly debunked, such as the notion that babies come from the cabbage patch, or that demons cause physical illness, is willful ignorance that can never produce anything good.
I argue not because I believe anything I say will ever convince anyone of my views, but rather that something I say may cause someone to start thinking for themselves, and they will convince themselves that their formerly held convictions were wrong. It is neither crime nor sin to be mistaken, but it is moral bankruptcy to refuse to admit when you're wrong.
I argue because I believe that a world without the divisiveness of religious factions, without all the griefs and calamities religious conflict has produced, would be a better world to live in than the one we have now. The world to come is the world our children, grandchildren, great-grandchildren (and on and on) will have to live in, and I only want the best possible world for them. Your god does not seem to want such a world for them; your god seems content to let this planet and its inhabitants self-destruct. Such a god, if existent, is a loathsome god. To believe in such a god is to acquiesce to the destruction of this planet. I am morally compelled to argue against belief in self-destruction.
I argue to protect your right to believe such nonsense, but at the same time it is imperative that I argue to prevent you from acting on those beliefs. Everyone has the right to believe any way they want, but no one has the right to behave any way they want, especially when such behavior is detrimental to the welfare of human beings. Acting on certain religious beliefs, such as calling for the execution of anyone who does not subscribe to a particular religious belief, is one example of such detrimental behavior.
I argue because I believe in people, not gods. I argue because it is the morally defensible position to take, that the welfare of my fellow human beings is more important than attempting the impossible task of satiating the ego of some mythological creature.
I argue because I am morally compelled to champion reason over superstition, to champion knowledge over willful ignorance, to champion understanding and compassion over blind faith and indifference, to champion justice over injustice, to champion love over hate.
I think that these are all good reasons for debating religion and theism, but as I make clear above a person has to be sincerely interested in learning or listening before any of this can have a chance to make an impression. Some religious theists, I'm sorry to say, are already so convinced of their "insightful" ideas that they don't care what real atheists might have to say about themselves — clearly they are too deluded to know their own minds, right? So they'll just keep repeating the idea that atheists have no business debating religion or theism.
Could it be a coincidence that if they succeed in getting atheists to believe this, then they would thereby suppress a significant source of questions about, challenges to, and critiques of their beliefs? Perhaps, but I have trouble believing that.