Here is a summary of my point in these two posts below:
I just feel you cant say you dont believe something that you had no concept of even existing.
For example, if I bowed my head to pray as an athiest, and started talking, who would I be talking to? Prayer is usully a conversation, so like in any, i need to have a concept or belief in the person I am praying to even though he doesnt exist. Likewise in stating you lack belief in something. "God" is not just a word. God can mean anything then. If you had a concept of God (or God exists) its easier to understans who you lack belief in.
Then again, if you lack belief in it, is that the same as saying he does not exist to an athiest?
I think this is the crux of the issue. Thanks for bringing this up.
There is no requirement for the understanding of a concept, as the only requirement for atheism is to be without the belief that God exists. Those that have not ever heard of the concept of God are certainly without a belief in the existence of God or gods, as they have no reason to hold one. Again, the term "lack" merely means to be without something in this context. It is not saying an insufficient amount of belief.
In regards to your example, an atheist who bows his head to pray would be doing so fraudulently. I'm not sure why you would think otherwise. An atheist is one who does not believe that God exists. Thus, an Atheist would have no reason to pray, as God would merely be a vague concept to him/her.
You go on to say this, "Prayer is usually a conversation, so like in any, I need to have a concept or belief in the person I am praying to even though he doesn't exist." Now, this clearly is illogical. You are contradicting yourself. An Atheist is one who does not believe in the existence of God. Thus, an Atheist is not going to pray because, to the Atheist, God is merely a concept invented by humans ... God doesn't exist apart from the linguistic concept to one who does not believe.
Your final point, "if you lack belief [in God], is that the same as saying [God] does not exist to an atheist", is almost right on. Saying that a person "lacks a belief in the existence of God" is saying that the individual "does not believe in the existence of God" or that they "don't think that God exists in reality". If you think that God exists, then you "believe in the existence of God". If don't think that God exists, then you "lack a belief in the existence of God".