If you are talking about the words themselves, then Atheism came second – because it was the opposite of theistic thought.
However, if you are talking about the nature of theism and Atheism and their meaning, then Atheism was first.
We are born without any beliefs in gods. We are born Atheistic. That is why theism goes to work on young children to make them believers before they become critical thinkers – to establish a pattern of gullibility.
It is important to note that the gullibility of a child may be an evolutionary survival skill. It was important for children to trust everything their parents said in the hunter/gatherer stage of our human development. The child had to trust the parent about poisonous plants and dangerous animals. This survivor gullibility persists today when we tell our children that something is hot and not to touch it or that something will bite them. Our gullibility may very well be biologic and a survival skill – helping to mold religious beliefs.
Early man (before Homo sapiens) was Atheistic until he started to explain away his fears by creating supernatural beings and gods. Early primates (and modern primates) are Atheistic – they have no religious beliefs or gods.
Atheism came first and we are all Atheists at birth. This refers back to my earlier discussion about the evolution of religion and the possibility, based on current research, that religious though may be a result of evolutionary modifications and certainly memetics in addition to genetics.
This of course brings up an issue for Christian doctrine. If the only way to heaven is to hear about and accept Jesus as your personal savior, then children that die do not go to heaven. Stillborns, mentally retarded, feral children, and isolated societies where missionaries have not reached are all going to hell.
The Christian will attempt to explain this through the “age of accountability” but that is mentioned nowhere in the Bible. Apologists invented the “age of accountability” in order to justify theological discrepancies and a major immorality issue with that theology. Even if the theology does not support it, no one wants their god to be responsible for sending babies to Hell for eternity.
What is more immoral than condemning children to hell because they don’t believe in your god? The theologians realized this and needed an apologetic to explain this and turn the immoral doctrine around.
Oh, the woes of monotheism and how everything becomes the fault or creation of one god instead of having a “bad god.” Even Satan cannot take the blame for sending these youngsters to the pits of Hell.