Anyone who asserts there is no God is an atheist, just as anyone who asserts there is must be called a theist. Anyone who takes up ground between these two groups is an agnostic.
Historically, polytheism has been the prevalent religion of ancient civilizations such as Egypt, Babylon, Greece, Rome, tribal animists as well as the dominant religions of the Far East. Ancient people lived closer to the time of the Creation events and could see the early earth as it existed. In polytheism, I think that people have invented their own gods but they acknowledged that deity existed.
Atheism has been tiny through much of history and remains a small minority position today. Communist USSR and China have been strongholds of atheism.
People have known and do know that there is a God. They may make a fuss about His identity and attributes but people generally acknowledge that God is out there.
In the Passover and Lord's Supper meals, there is a remembrance of great works of God in the past. These remembrances were started when those great works of God were new. They have continued annually or more often since then.
"Some seek to remember, some seek to forget". It seems that atheism seeks to forget.