"What a fool believes no wise man has the power to reason away."
Arguing God with atheists is something I like to do - a lot. Both in person and over the Internet. It's fun and I've learned some things along the way: atheists come in different flavors. Not all of them are overt God haters. Not all of them are overtly anti-religion. Some are rabid leftists. Some are rabid anti-leftists. Some are quite intelligent. Some are very stupid. Some of them are somewhat likable... All of them are fools.
Never forget this: however many degrees they may or may not have, or say they have, when arguing God with an atheist, you're dealing with a fool. So, don't be a fool yourself. You and the atheist are not on the same level. While you may not be on a pedestal, he is definitely in a ditch.
When debating with an atheist, until there are compelling reasons otherwise, treat him as an individual.
Always let the atheist get nasty first - though you don't have to get nasty. When he does get insulting, then you're allowed to respond in kind. The choice is yours. It's been my experience that they usually do get nasty - and pretty fast. Most of them ARE bitter, nasty human beings. This is helpful though, because it means your atheist is emoting rather than thinking. Like a boxer who starts losing his cool in the ring, they become easier to beat when they get mad.
AVOID CITING THE BIBLE! Atheists reject the Bible out of hand and there is no common ground there for you and the atheist to come to terms. They see the Bible (or the Koran, as they hate Muslims too) as only a book of mad fairy tales for gullible children. Your problem is that it IS a book. Books can't be questioned. But even if you were to open a Bible and it started talking to them, they'd still find some reason to reject it. It's foolish to appeal to religion when arguing with an atheist. You must find other ways of beating him.
However the argument may develop, you must maneuver the atheist into philosophy. At its core atheism is a futile, irrational and anti-religious, religious system. It IS a cult - much as atheists resent that definition. Philosophy is a mental martial art that, when used correctly, will send the atheist crashing to the floor - judo like. This takes some experience as it involves setting them up, getting them to commit to a position, then using the momentum of their own reasoning to spring your trap on them.
At its core, atheism is a logical fallacy called, "denying the consequent". Denying the consequent proceeds thus: A. "If God exists then I would see Him." B. "I do not see Him." C. "Therefore, God does not exist." This isn't to say that many religious people don't embrace philosophical fallacies too, but ALL atheists do. Atheists like to posture as paragons of reason and intellect. The truth is quite the opposite: on the matter of God at least, they think illogically and often childishly.
Atheists also resort to a myriad of other logical ruses: tautologies, guilt by association,the masked man fallacy, the ad hominem fallacy, the genetic fallacy, the argumentum ad ignorantiam, the fallacy of the undistributed middle, circular reasoning, begging the question, argumentum ad verbosum, straw man arguments, proof by assertion, false appeal to authority, lying....
They also like to confuse "evidence" (of something that supports *their* pov) with "proof". At the same time they will deny any evidence *you* bring to the table. They'll always demand that you need more evidence to "prove" your case. If you cite a book about a great miracle in Portugal, they will say the book is a pious fraud. If you bring pictures or video of the miracle to their attention, they will say, "they were doctored". And if they should see the miracle with their own two eyes they'll say, "it was an optical illusion." There isn't enough evidence in the known universe to convince such people.
They almost always, if not always always, resort to denigrating the theist (you?) as some sort of intellectual dwarf compared to themselves. They *love* to think of themselves as intellectuals. Religious people on the other hand, and especially Christians, they deride as mental midgets. Don't be surprised when they start saying how unintelligent you are. They always do. The more benign atheist may not overtly say that, but he *will* imply it. You are a childish fairy tale believer who imagines there's an invisible "sky daddy"... and is sooo unworthy of their valuable time (but they'll still spend *a lot* of time responding to you).
I mentioned "springing the trap" on them. This is very important and there are a number of ways of doing it. I like to dangle leading questions, in the hope the atheist will bite, and get it wrong. When they do, I pounce on it - then pound them with it. If they come back I just keep pounding them with it - while mocking their "jeenyus". Once you're able to do this they vamoose. Another way is to catch them in a grammatical error. For some reason this really infuriates them. If they say "your" when "you're" is called for, you can mock them for it, (because they all believe themselves highly intelligent) and they soon and angrily slam the door on their way out.
I always like to ask them,"can you prove there is no God?" Of course they can't - and it makes them angry to be forced into that corner. They'll reply, "No... it's up to you to prove there IS a God!" In fact, I can't recall one who's frankly admitted they can't prove there isn't a God. They'd rather be burned at the stake. Anyway, this flies in the face of accepted science and historical revisionism: those who contradict the accepted story are the ones who must supply their proof/evidence refuting it.
Furthermore, saying "There IS NO God!" is a declarative statement - no less than saying, "there IS a God!" Declarative statements demand back-up. The atheist simply tries to win by default, thinking, "If theists cannot prove their case then I win and there is no God who'll condemn me to hell."
Basically, the atheist is a spoiled child rebelling against a, "'sky daddy' that doesn't exist" - but whom he hates anyway.