... [M]any atheists do not accept faith as a valid way of knowing. I am such an atheist who also rejects the proposition that faith is a valid way of knowing. I believe that knowledge is derived through the senses and evaluated through reason.
For sure faith--belief without evidence--is not a valid way of knowing. To know is to be able to provide justification for one's belief. I can say I know that X exists if I provide good reasons and evidence to show and persuade others that X in fact exists. Faith, which is mere belief, in no way implies that what is believed in is true.
Schick and Vaughn explain this much more clearly than I can:
"Faith," as it is ordinarily understood, is "belief that does not rest on logical proof or material evidence." To believe something on faith is to believe it in spite of, or even because of, the fact that we have insufficient evidence for it.... In the case of faith, the gap between belief and evidence is filled by an act of will--we choose to believe something even though that belief isn't warranted by evidence. Can such a belief be a source of knowledge? No, for we cannot make something true by believing it to be true. The fact that we believe something doesn't justify our believing it. Faith, in the sense we are considering, is unquestioning, unjustified belief, and unjustified belief cannot constitute knowledge.
...To say that you believe something on faith is not to offer any justification for it; in fact, you are admitting that you have no justification. Since believing something on faith doesn't help us determine the plausibility of a proposition, faith can't be a source of knowledge. (p. 110-111)
... If belief is rational, there is some reason to hold it, and if there is some reason to hold it, there is some evidence in its favor. Since belief without evidence is not rational, faith is not rational....[O]nly rational belief can yield knowledge. So faith cannot be a source of knowledge. (p. 112-113, emphasis original)
(Theodore Schick, Jr. and Lewis Vaughn. How to Think About Weird Things: Critical Thinking for a New Age, 2nd ed. Mountain View, CA: Mayfield, 1999. )