While wandering around the internet, I encountered an Atheist who claimed that "a belief in NO god" was not a correct definition and that atheism actually includes agnosticism: since lacking a firm belief in God, meant that an Agnostic cannot be a Theist. Why he thought that including Agnostics buttressed his position eludes me; since the Agnostics are likely stumbling around thinking "I do not understand, why am I here?".
I claim that both theism and atheism are united in having a strong conviction, while agnosticism is characterized by doubt. There may be a weak form of atheism that is nearly indistinguishable from agnosticism; but let's just call them Agnostics, since they cannot make their mind up (a key trait of agnosticism).
To make this clearer, let's consider a Schrödinger's cat type of thought experiment, where the metaphysical cat might be the "concept of godhood": we are presented with a box that we cannot open and told it contains this cat. What are the responses?
Theist: I believe that this box contains a cat that is alive.
Atheist: I believe that this box never contained a cat; but if it had, the cat is now dead.
Agnostic: I have no way of knowing the contents of this box at this time, so I cannot form an opinion.
There is a short spectrum of agnosticism that runs from weak (I do not know) to strong (I can not know). As Chidi Anagonye said in The Good Place: "I don't know, please don't ask me that, my stomach hurts".