Although I think you have the timeline flipped there. Atheist is a 16th century word with a pretty clear etymology that means "without God", or more literally "not a Theist" - AKA "lack of belief in God", whereas "against God" would be constructed as "antitheist" from those same roots. Agnostic is a 19th century word that from its etymological roots would be synonymous with atheist, if perhaps slightly more broad as to applying to not just all personable God's but all unknown powers in the universe.
The split to atheism being considered a belief against God and agnostic being a lack of belief is a more recent development, and is a evolution that is still incomplete, probably even reverting in most modern usage. Possibly because the language roots are so obvious and at odds with that context driven interpretation that people unfamiliar with the conflict will see "atheist" and immediately think "that must mean lack of faith"
Got it, I think.
Although I think you have the timeline flipped there. Atheist is a 16th century word with a pretty clear etymology that means "without God", or more literally "not a Theist" - AKA "lack of belief in God", whereas "against God" would be constructed as "antitheist" from those same roots. Agnostic is a 19th century word that from its etymological roots would be synonymous with atheist, if perhaps slightly more broad as to applying to not just all personable God's but all unknown powers in the universe.
The split to atheism being considered a belief against God and agnostic being a lack of belief is a more recent development, and is a evolution that is still incomplete, probably even reverting in most modern usage. Possibly because the language roots are so obvious and at odds with that context driven interpretation that people unfamiliar with the conflict will see "atheist" and immediately think "that must mean lack of faith"