The Dictionary says: "a·the·ist, n. a person who denies or disbelieves the existence of a supreme being or beings."
Note that it explicitly says nothing at all about the degree of certainty, nor about whether the belief is held as an article of faith that is not subject to challenge (i.e, as a fundamental axiom taken as true without proof.)
The idea that atheism requires any faith is a calumny not supported by the dictionary, nor by the usual understanding self-professed atheists typically have of their own philosophical position.
But you are, of course, free to understand the term as you wish.
So, by that standard, a frightened Catholic priest that denies that he believes in God when a Communist Commissar puts a gun to his head is an "atheist".
By that standard, Bill Clinton, who stated in no uncertain terms that he "did not have sex with that woman, Ms. Lewinski" was a "faithful" husband.
The dictionary definitions of "atheist" and "faithful" assume that the person in question actually says what he means and means what he says in regards to his religious beliefs or his extramarital conduct.
Therefore, Clinton, the self-described faithful husband is unfaithful regardless of what words he mouths and the self-described atheist who is not certain whether God actually exists or not is an agnostic and not a true atheist no matter what words he mouths.
Of course, we can now start debating what the meaning of the word "is" is.