ag·nos·tic
/aɡˈnästik/
noun
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.
a·the·ist
/ˈāTHēəst/
noun
a person who disbelieves or lacks belief in the existence of God or gods.
An atheist does not believe in a god or gods as their is a lack of evidence to their existence.
An agnostic believes there is no way to prove a god or gods exist and therefor are unknowable.
If the Great Tribulation were to start it would be physical evidence in favor of not only the existence of a god, but the God of the Bible. An atheist may change belief. An agnostic would dismiss the evidence as agnostics believe any evidence of a god is not possible in the physical universe.