We were atheists before we were theists both as a species and individually:
"All children are born atheist. They have no idea of God" - Baron d'Holbach
To my recollection, my parents never spoke to me about any gods, and we didn't do the things characteristic of theistic families such as pray at meals, read holy books, or attend weekly gatherings. We had no extra respect or disrespect for clergy, and no concepts such as sin or blasphemy in our consciousness. We were (and are) atheists.
I don't see a response to theism there. There wasn't really much acknowledgement that it existed. Eventually, theists came along and try to talk me into joining their religions. I said, "No thank you." That might have been my first response to theism.
You might be thinking linguistically rather than ideologically.
There are things that came earlier but were named later, such as acoustic guitars. Before the advent of electric guitars, there were just guitars. They were all acoustic and there was no more need to identify them as acoustic than to identify them as strung or tunable. Likewise with snow skiing and Classic Coke, which were skiing and Coke before water skiing and New Coke came along. Obviously, Word War I didn't get that name until there was a World War II.
Atheism and theism have a similar relationship. Atheism is older than theism, although the word and even the concept may be newer.
This is not the case with antitheism, which is definitely a reaction to theism. Whereas atheism exists with or without theism in the world, and whether it has a name or not, theism must precede antitheism.
Do you disagree with any of that?