This is an excerpt from a longer interview by Scott Burdick (here).
The book Richard Carrier is talking about is Why I Am Not a Christian. For a summary of his argument, check out his essay of the same title, from 2006. Here's a sample:
1. God is Silent
If God wants something from me, he would tell me. He wouldn't leave someone else to do this, as if an infinite being were short on time. And he would certainly not leave fallible, sinful humans to deliver an endless plethora of confused and contradictory messages. God would deliver the message himself, directly, to each and every one of us, and with such clarity as the most brilliant being in the universe could accomplish. We would all hear him out and shout "Eureka!" So obvious and well-demonstrated would his message be. It would be spoken to each of us in exactly those terms we would understand. And we would all agree on what that message was. Even if we rejected it, we would all at least admit to each other, "Yes, that's what this God fellow told me."
Excuses don't fly. The Christian proposes that a supremely powerful being exists who wants us to set things right, and therefore doesn't want us to get things even more wrong. This is an intelligible hypothesis, which predicts there should be no more confusion about which religion or doctrine is true than there is about the fundamentals of medicine, engineering, physics, chemistry, or even meteorology. It should be indisputably clear what God wants us to do, and what he doesn't want us to do. Any disputes that might still arise about that would be as easily and decisively resolved as any dispute between two doctors, chemists, or engineers as to the right course to follow in curing a patient, identifying a chemical, or designing a bridge. Yet this is not what we observe. Instead, we observe exactly the opposite: unresolvable disagreement and confusion. That is clearly a failed prediction. A failed prediction means a false theory. Therefore, Christianity is false. ...
Right from the start, it fails to explain why believers disagree. The fact that believers can't agree on the content of God's message or desires also refutes the theory that he wants us to be clear on these things. This failed prediction cannot be explained away by any appeal to free will--for these people have chosen to hear God, and not only to hear him, but to accept Jesus Christ as the shepherd of their very soul. So no one can claim these people chose not to hear God. Therefore, either God is telling them different things, or there is no God. Even if there is a God, but he is deliberately sowing confusion, this contradicts what Christianity predicts to be God's desire, which entails Christianity is the wrong religion. Either way, Christianity is false.