In dialogues with atheists, I regularly see theists pose the following inquiry:
If you don't have faith in God, then what do you have faith in?The most typical answers include rationality, science and humanity. While I find nothing truly objectionable in these responses, it's my opinion that the most appopriate response is in fact, “Nothing. I have no faith.”
The problem is that a believer's faith and an unbeliever's faith are completely different. When a believer says they have faith in God, they mean that they believe in his existence without sufficient evidence, that they trust God to always do what's right, or both. When a nonbeliever says they have faith in rationality or science, they mean they think that's the best method to discover the truth about the universe, and when they say they have faith in humanity, they mean that humans must solve their own problems and that they have hope they in fact will, but it's never unbounded confidence like believers put in their deity. These two religious and two secular varieties of faith just don't truly overlap. Support for the scientific method and recognition that we must address our problems without divine assistance are not at all equivalent to belief without evidence and absolute trust in an invisible being, even though we often call them all by the same name of faith.
So in the sense that theists probably mean when they ask the question, most atheists simply have no faith whatsoever, but I don't really fault them for providing a seemingly more positive and optimistic answer than what I recommend. For my own part, however, I prefer to answer as honestly as possible and to challenge the assumption of the believer that I must have faith like they do.