https://en.wikipedia.org/wiki/Atheism

Atheism is, in the broadest sense, the absence of belief in the existence of deities.[1][2][3][4] Less broadly, atheism is the rejection of belief that any deities exist.[5][6] In an even narrower sense, atheism is specifically the position that there are no deities.[1][2][7]

As you can see Atheism is the belief that no such Deity exists. Theism Is the belief that such a Diety does exist. Agnosticism is to not have the evidence to believe either with certainty.