Well, I don’t think that word had been invented yet, but there were phrases like “scoffer” and “unbeliever”.
Then sometimes they were just called “fools”:
‘The fool says in his heart, “There is no God.”’ - Psalm 14:1
At the individual level, there were atheists/agnostics. But I don’t recall any OT clan or society that was atheists/agnostics.