In this discussion I'm going to defend two basic contentions:
I. There are no good reasons to think that theism is true, and
II. There are good reasons to think that God is a myth.
Let's look at the first major contention, that there are no good reasons to think that theism is true. Theist philosophers have tried for centuries to prove the existence of God. But no one has been able to come up with a convincing argument. So rather than attack straw men at this point, I’m going to wait to hear any answer to the following question: What is the evidence that theism is true?
Let's turn then to my second basic contention, that there are good reasons to think that God is a myth.
Now I'm not claiming that I can prove that God is a myth with some kind of mathematical certainty. I’m just claiming that on balance the evidence is such that God is a myth is more plausible than not. Let me present, therefore, five reasons why I think it’s more plausible that God is a myth than that theism is true.
1. Creation myths explain why there are gods. Have you ever asked yourself why anything at all exists, or where the universe came from? Typically, theists have said that the universe is the creation of God. But surely this is unreasonable. Just think about it for a minute.
Creation myths are a part of human culture. Throughout human history, different explanations involving gods and supernatural agency have been passed down through oral and written traditions. Creation ex nihilo is a common feature of many belief system, including those of ancient Egypt, of Hinduism, and if many animistic cultures throughout the world.
Elizabeth and Paul Barber, in their work When They Severed Earth From Sky, talk about the Wilfulness principle. They argue that it may seem absurd now to think about a tree falling over as anything other than wind, but wind as it’s conceived now is a result of thousands of years of data collection parsed through genius minds. To the ancients, how could they have conceived of huge quantities of near-infinitesemal invisible particles acting in accordance with the fundamental forces of nature? Thinking in terms of wilful agents, and invisible agents at that, would be the only possible source of explanation.
H. and H. A. Frankfort, in their paper Myth and Reality posit: "The fundamental difference between the attitudes of modern and ancient man as regards the surrounding world is this: for modern man the world is primarily an "It"; for ancient man it is a "Thou." [...] An object, an "It," can always be scientifically related to other objects and appear as part of a group or a series. In this manner science insists on seeing "It"; hence, science is able to comprehend objects and events as ruled by universal laws which make their behavior under given circumstances predictable. "Thou," on the other hand, is unique. "Thou" has the unprecedented, unparalleled, and unpredictable character of an individual, a presence known only in so far as it reveals itself."
This doesn't necessarily exclude gods, but it does raise major problems for the idea of gods. The first problem is the sheer abundance of myths, so why should we privilege one mythic explanation above any other? Do we accept, as CS Lewis argued in Mere Christianity, that God solely revealed Himself to the Jewish people some 3500 years ago, and that the bible is the inspired word of God? That the commandment "Thou shalt have no other gods before me." is the one true instance of a correctly-identified god cautioning against all other imaginary gods?
Or perhaps, as J.L. Mackie notes in The Miracle Of Theism: "The advocate of one religion will now often allow that a number of others have at least some elements of the truth and even, perhaps, some measure of divine authorization." But this raises the further problem that Mackie immediately elucidates: "Carried far enough, this modern tendency would allow Christian miracles to support, not undermine, belief in the supernatural achievements of stone-age witch doctors and medicine men, and vice versa." He was talking in the context of miracle reports, but the same principle applies. Lewis advocated a view similar to this, but had the marker of the truth of other religions as his own.
Though, what would seem the most reasonable assumption, is that none of them are correct. That gods as explanations are sufficiently misguided as to not think of the myths as containing a literal truth about the nature of the beyond. Jerry Falwell claiming that 9/11 was God's punishment for the ACLU highlights just how it is that people can see God's hand in nature where there clearly is none.
The second problem, and the most significant challenge, is how to take prescientific ideas like God in light of modern science. As Stephen Hawking and Leonard Mlodinow put it in The Grand Design: "In the first two thousand years of scientific thought, ordinary experience were the basis for theoretical explanation [...] we began to find nature behaving in ways that were less and less in line with our everyday experience and hence our intuition." Invoking gods seem more and more archaic a thought the more that nature is understood. "Sire, I have no need for that hypothesis", the words of Laplace echoed today by top scientists of various disciplines.
Personal causation, too, has come under the lens of scientific inquiry. Personal causation isn't something removed from the physical, but an expression of it. Without the brain and without the physical systems throughout the body, it would make no sense to talk of personal causation. Thinking is what the brain does, and experiments such as the Libet experiment show that conscious awareness happens after decisions are made in the brain. As Dan Dennett pointed out in his debate with William Lane Craig, personal causation is a special kind of physical causation. Psychologist Stephen Pinker characterises this as "the mind is what the brain does".
Even if physical accounts of consciousness are lacking, there are two observational facts that really seal the relation between mind and brain. Observations have shown time and time again a link between particular brain activity and behaviour/experience. Damage regions of the brain and lose functionality. Manipulate the brain and alter experience. Out-of-body experiences can be induced with magnetic fields, as can altering moral decision making. Neurophilosopher Patricia Churchland has talked of one case where a tumour in a man's brain resulted in paedophilic sexual urges. Removing the tumour meant the cessation of those urges.
The second observation is that despite all the research probing the brain, there's no indication of anything other than brain activity being involved in cognition. Descartes, when he first posited mind-body dualism, identified the pineal gland as the interface between mind and body. Yet with all the possible opportunities to find anything outside the brain, nothing has yet shown up. It may be that there's something outside the brain, but the evidence is to the contrary.