Naturally, this observation relies on the definition of 'faith'.
There are different meanings and usages of this word. In a cynical sense, faith is but an excuse to do whatever one desires. Sometimes it refers to an innocuous belief about a trivial subject (I believe that to be the case...). Atheists may use the word to describe irrationality--that which has no proof, justification or reason. Or, more humorously, “Faith is believing what you know ain’t so.”
The online Webster dictionary has a range of meanings:
"2 a (1) : belief and trust in and loyalty to God (2) : belief in the traditional doctrines of a religion b (1) : firm belief in something for which there is no proof (2) : complete trust
3 : something that is believed especially with strong conviction; especially : a system of religious beliefs"
When it is asserted that atheists have faith, point 3 is in mind. Many atheists have a strong conviction about the non-existence of God or believe in the value of the empirical method, etc. At other times atheists are convinced of the integrity of researchers they quote or the methods they use. Another way to write this is to assert that atheists have a belief--a belief in their worldview. Belief is a synonym for faith. Webster's definition of belief is similar to faith:
"3 : conviction of the truth of some statement or the reality of some being or phenomenon especially when based on examination of evidence."
It is interesting that the definition of faith appends "a system of religious beliefs" and the entry on belief omits this phrase. The use of the word "especially" does not mean "uniquely". In fact, the third definition of faith includes the word 'belief'. And belief should be based on reasonable evidence. (For those more philosophically minded, the Stanford Encyclopedia of Philosophy has an in-depth article on belief--that attitude we have about what we regard as true).
This usage of the word(s) is important because when this claim is made it is not meant to be a vacuous idea. There is substance to the claim. Faith--conviction or trust based upon reasonable evidence--is common to all mankind because of creaturely finitude. It is meant to remind the detractor that the debate between Christians and non-Christians is between competing faiths--worldviews--between different evidences, proofs and justifications and the very philosophy of knowledge and justification itself.
It is unfortunate that too many Christians present faith in antithesis to reason. They are using faith in a way that traditional Protestant leaders and creeds have not. Reason is a tool. And its usage and limits are determined by the framework in which it is used and defined.
Yet it is hoped that atheists and others may realize that many a Christian (leader) do believe there are good and sufficient reasons for Christianity and Christianity's God. Although the faith of a Christian must include trust and rest in Christ, faith as such is not irrational by this definition. When faith is defined as irrational at the outset of a discussion (thus defining out of existence intelligent Christians), it must be vigorously denied.
All men and women believe in truth as they understand it. Atheists have such a faith. And so do Christians. The debate between the two is over which faith is best justified.