I came across a section in his "Belief in God" chapter that reminded me of points made in my post -- which isn't surprising, given that (1) Shermer's arguments are fairly obvious, and (2) almost certainly Shermer and I have read the same ungodly books by Dawkins, Hitchens, Dennett, and other religious skeptics.
Have a read:
The problem we face with the God question is that certainty is not possible when we bump up against such ultimate questions as "What was there before time began?" or "If the big bang marked the beginning of all time, space, and matter, what triggered the first act of creation?"
The fact that science presents us with a question mark on such questions doesn't faze scientists because theologians hit the same epistemological wall. You just have to push them one more step.
In my debates and dialogues with theologians, theists, and believers, the exchange usually goes something like this for the question of what triggered the big bang, or the first act of creation.
God did it.
Who created God?
God is he who needs not be created.
Why can't the universe be "that which needs not be created"?
The universe is a thing or an event, whereas God is an agent or being, and things and events have to be created by something, but an agent or being does not.
Isn't God a thing if he is part of the universe?
God is not a thing. God is an agent or being.
Don't agents and beings have to be created as well? We're an agent, a being -- a human being, in fact. We agree that human beings need an explanation for our origin. So why does this causal reasoning not apply to God as agent and being?
God is outside of time, space, and matter, and thus needs no explanation.
If that is the case, then it is not possible for any of us to know if there is a God or not because, by definition, as finite beings operating exclusively within the world we can only know other natural and finite beings and objects. It is not possible for a natural finite being to know a supernatural infinite being.
...The burden of prooof is on believers to prove God's existence -- not on nonbelievers to disprove it -- and to date theists have failed to prove God's existence, at least by the high evidentiary standards of science and reason.
So we return again to the nature of belief and the origin of belief in God. I have built a strong case that belief in a supernatural agent with intention is hardwired in our brains, and that the agent as God was created by humans and not vice versa.