Can I reject a god if there was not a god to reject in the first place?
What I am rejecting then is not a god but the notion or the concept of a god. Theists want to know how you can reject a god or God by positing its (or His) existence first by naming it then asking you to how you can reject it when it exists. What the theists mistake for a god is its name.
There is no proof of existence of a god other than by its name and the actions based on the belief of its existence, not on actual proof of its existence.
I do not believe in God, a god or gods but I do see the concept of God, a god and gods all around me in the form of human behaviour and nothing more.
An atheist, then, should not be a rejection of the existence of a deity, anymore than I should not reject the existence of, say, Donald Trump. By rejecting existence of Donald Trump I would be considered a lunatic. This kind of thinking is what theists uses against an atheist’s rejection (the “how can you not believe in the existence of God?! That’s absurd!” argument).
The very term atheist relies on (or presumes) a positive term of (or a presence of a god) to reject in the first place. The terminology dupes us into a double take on whether there is a god to reject or not. Agnostics, in return, tried to play the same game, being drawn into a competition of verbal trickery. This will no longer do.
Undoubtedly, I believe in the existence of the concepts of deity and deities. This is what we like to do as humans. It is natural for us to broadly paraphrase Mary Midgley to want to be given a reason to live. We are scared to live in a meaningless world as Jean-Paul Sartre might have said.