You believe in sky fairies!
Mentioning "sky fairies" or "invisible friends" is a sure sign of a troll who has made no attempt to understand what it is that Christians believe in. Other good terms of abuse are "men in dresses" (actually, having gatecrashed the Vatican changing-rooms, I can reveal that most priests wear trousers under their vestments) and the old favourite "all made up by bronze-age goatherds", as if it were not possible for philosophers and goatherds to live at the same time. Remember, that Richard Dawkins wrote a thesis on the behaviour of chickens, and may thus be described as a plastic-age chicken-herd. That's him sorted.
"You believe in talking snakes."
The "talking snake" refutation of all Christian thinking goes as follows: "Snakes can't talk. Therefore the Bible is fiction. So Jesus never existed. I win."
Either your atheist has read the Bible as far as the Garden of Eden story and then stopped, or else has lifted this argument straight from the works of Richard Dawkins. It doesn't occur to him or her that a God who made Heaven and Earth could easily endow a snake with the power of speech (or perform any other miracle, such as the Virgin Birth). That's what omnipotence is all about, you see. Nor does it occur to him that it is not necessary to believe in the literal truth of the Genesis story to realise that it has a meaning. You know, the Fall of Man, that sort of thing.
Worse than that, your atheist believes that Christians DO NOT KNOW that snakes can't talk. Not being trained scientists (although many are), Christians are not able to understand this simple fact. It must be true that Augustine and Aquinas - of whom your atheist probably hasn't even heard - went through life talking to snakes and wondering why they didn't reply.
"Bad things happen. So God doesn't exist and I hate Him."
This is of course the "Stephen Fry" version of atheism. Stephen Fry is not entirely dim - a 2.1 from Cambridge must be nearly as good as a 2.1 from Oxford - but he does seem to be woefully ignorant. After a lifetime of reading quiz answers off little cards and getting an undeserved reputation for being the biggest genius who ever lived, Fry is apparently unable to deal with any concept that requires more than a little card to explain. Again, he thinks that nobody before him has ever worried about why God permits death and disease, or even about why He permits every single radio and television programme to be infested by a certain charmless bore who thinks bad language is funny.
"You eat shellfish, and so it's perfectly OK for me to have sex with anyone I wish.
Yes, it's another poorly-informed atheist, or maybe just a troll or an obsessive. No, Christians aren't bound by the dietary laws of Leviticus, they've been explicitly superseded by the New Covenant. Yes, murder, adultery, theft, and the like are still considered to be wrong. I don't think I can face explaining this one again: come back when you've read some of the New Testament.
An atheist (reading Eccles's blog, as it happens).
One final argument goes something like this: "All right, then. The Bible isn't historical, because all the people who contributed to it believed in God. Why don't we have some accounts of Jesus's life, death and resurrection written by eye-witnesses who were atheists? You suppressed those, didn't you?" Somehow, I think it's not really necessary to answer that one.