Unless he/she knows all there is to know, an “atheist” is most likely an agnostic without having reached that conclusion yet.
It goes like this, in an interview with someone who professes to be an atheist:
Q. Do you know all there is to know?
A. No.
Q. Do you know half of all there is to know?
A. No.
Q. Let’s say you do. Is it possible that you could find God in that half of what you do not know?
A. Yes, it’s possible, though I don’t believe so.
Q. Then, in essence you are an agnostic since you acknowledge it is possible for God to exist, correct?
A. ?
The proper answer to this question is "Since I have no belief in any deity while not affirming that no deities exist, I am an atheist of the weak variety rather than a strong atheist. This is distinct from being an agnostic, is it not, who when asked if God exists answers, 'Maybe'?"
A better answer than "?", I'm sure you'll agree.
By that reasoning, perhaps Christians would also be agnostics, in that if they were to, in the after life come face to face with Set, Moloch, or Dagon (as opposed to St. Peter) they would then have enough information to change their mind.
That Q&A works on everything. It turns honest believers into agnostics. Just replace God in the 3rd question with the phrase ‘proof that God doesn’t exist’. It’s not a terribly illuminating line of questioning in that respect.