The Merriam-Webster definition you're using (like many dictionary definitions of involved philosophical topics) is cursory and misleading. Anything that covers the subject in any depth will tell you that agnosticism involves what a person can know, and atheism what they believe (or not believe, as the case may be).
Agnosticism just says that the existence or non-existence of any god or gods is unknowable. An atheist simply doesn't believe that any gods exist. So, in fact, both an atheist and a theist can also be an agnostic. An agnostic atheist
just takes the attitude that no amount of evidence can be used to support the existence of any gods, so it is pointless to believe that they do. An agnostic theist
says that although there is no evidence for the existence of any gods, they choose to believe by faith.
The question to ask is which of the following two have more "faith": the atheist who sees no evidence in god and so does not believe in it, or the theist who also sees no evidence and does. I'd argue that one takes much more than the other.