I've had lots of debates with Christians over the years, and I've found that the best way to make a point about atheism is to take the issue out of the current culture and into a culture that never knew of Christianity or Judaism.
When they say that being an atheist isn't reasonable, because of X Y or Z reason, simply use this argument:
What about atheists among the Aztecs? Virtually every known culture has had atheists, we know this because the religious texts of every culture that has any texts discusses atheists. Indeed both the New and the Old Testament discuss atheists. Given that every culture has had atheists, what about atheists in a culture like, for example, the Aztecs?
The Aztecs believed in hundreds of gods. They believed that many of their gods required human sacrifices in order to be appeased. Now, what about some Aztec 600 years ago who had no way to have knowledge of any other gods, but who did not believe in any of the Aztec gods?
Was it reasonable for that Aztec to be an atheist?
If they don't like the Aztec thing, go with the ancient Greeks. It's a little less good, because its theoretically possible that Greeks could be aware of the Jewish god, but its also good cause we know there were Greek atheists.
The argument is the same. So what about Greeks living hundreds of years before Christianity, who knew nothing about the Jews, but who knew that none of the Greek gods were real? We know for sure that such people existed, they wrote "books" and made their arguments against the existence of the Greek gods.
Given that they had no way to know of any other gods, isn't it reasonable then that they were atheists?
If they say no then it goes to a discussion of why they think that its better for people to believe things that they know to be false, and you go down that road...
If they say yes, then they have admitted that it is reasonable to be an atheist under certain conditions, its just that they think that the god they believe in is "different".
You then point out that atheists have been treated relatively equally by religionists in all cultures, that the ancient Greeks said the same thing about atheists then and Christian do now, and the same for virtually every culture, etc., etc. and its just special pleading to argue that in this case "its different", etc., etc.