a·the·ist - a person who disbelieves or lacks belief in the existence of God or gods.
ag·nos·tic - a person who believes that nothing is known or can be known of the existence or nature of God or of anything beyond material phenomena; a person who claims neither faith nor disbelief in God.
Atheism answers the question, “Do you believe God or gods exist?”
The evidence for the existence of all 4000 of the gods and goddesses people have worshipped from the beginning of recorded time is identical:
Since Christian God, Muslim God, the Greek gods, the Roman gods, the Nordic gods, the gods from shamanistic cultures, and so on are all validated subjectively, they each have an equal likelihood of existing.
One could easily make up stories of a newly invented god and convince others to believe them. And, this would result in the same spiritual “feelings” that validate the existence of established gods (think Scientology or Moonies). Therefore, any absurdity conjured up by the human mind has the same likelihood of existing as any of the other 4000 gods and goddesses worshipped by humans.
So, based on the unimaginably low probability that established or newly invented gods are real, the atheist makes the claim, “I don’t believe god/s or goddess/es exist.”
Agnosticism answers a different question, “Can we know if God or gods exist?”
In a universe of infinite time and space, anything is possible. We truly cannot know for certain that god/s or godess/es do not exist. We cannot know for certain that fairies, smurfs, and leprechauns do not exist. It is possible that invisible horses with Snoopy heads exist in a place called Woowoo. The probability that this scenario is true is astronomically low. However, it is the exact same likelihood that Thor and Odin exist in a place called Valhalla or that the Christian and Muslim gods exist in a place called Heaven.
So, the agnostic states that he/she cannot know if god/s or goddess/es exist. And, the atheist states that, based on probability, he/she does not believe god/s or goddess/es exist.