Needless to say, I have found that as with any subject you bother to educate yourself in it seems that Mythology should eventually lead you to doubt the validity of a God, or specifically the god you were brought up with if you are a theist. Its amazing that someone could not come to this conclusion, when you read how people in these myths believe or depend on various gods based on their location among other factors. Thats is people close to the ocean believe in Poseidon, while those that live inland may believe more in another god like Zeus or Apollo, or hunters may be more believers in Artemis.
What you will also realize in these reading is that Zeus, Hera, Dionysus or whatever other Greek god you care to name all have very personal serious issues. Yet even with all this craziness going on these gods were praised. In the same way we atheists wonder why modern day theists adhere to beliefs about their gods like Yahweh or Allah. All these gods that people believe in today have seriously screwed up ideas of right and wrong, and it just takes a little look into myth to understand why these ideas are so out of place in modern society. These Greek gods were just the Greeks way of trying to enforce societal norms on the people of the time. However, these societal norms of extreme gift giving (i.e. you daughter) or going to war as you feel insulted are not accepted anymore. We have moved on and we need to move on from these other ridiculous stone age beliefs held by Muslims, Christians, etc. In fact we need to move on even from crazy modern day beliefs like Scientology, Mormonism etc.
So when an atheist says to a theist, "you don't believe in Zeus as I do, however I just don't believe in one more god." Just remember, there is a lot more to this than just simple mockery.