CTD:
I think that there is a fundamental problem with the question or challenge that you pose.
The nature of proof is that it gives us irrefutable evidence that something is a specific way. There are many things that you can speculate upon, but will never be able to get proof of either way.
Proving that God exists is just as impossible as proving that God doesn't exist. We lack the capabilities to gather the evidence. Now, if we can't prove it either way, yet God must either exist or not exist, how can we determine which of the two options is more sound?
We can only speculate on the likelihood of the existence of God through our own personal experiences. The most common answer that is given as to why people believe in God is the way they feel. Some people are very spiritual, and they are comforted and helped through their lives by a close relationship with God, which I think is very healthy. On the other hand, many people don't care for the idea of God, and they are content living their lives without religion of any kind. Even though one of these people is right and the other is wrong, neither is confused or deluded. The absolute fact of the matter is that we cannot objectively know that God exists. That's an integral part of the Christian (and other) Faiths. To follow religion, one needs to whole heartedly accept that their religion is the truth despite the fact that there is no evidence and most likely will not be any evidence.
Now, to address your challenge specifically, you cannot find a claim that is more absurd, but you can find many that are equally absurd. The statement is false, end of story. There is no person who can claim they have knowledge of the nature of God, whether it is regarding his existence or non-existence. The real substance of what you are asking is not about whether or not God exists, but about whether or not we can have knowledge of it.
If you actually mean to say that you are looking for a belief that is more absurd than to believe that God does not exist, I can give you countless examples.
The world is flat.
Masturbation will make you go blind.
A planet called Vulcan exists somewhere between mercury and the sun.
Phlogiston makes combustion possible.
These are just a few beliefs that were actually held by intelligent people in the relatively near past. The reason that all of these beliefs are more absurd than the belief that there is no God is that we have proven that they are false. We know that the earth is round and that excretion of sperm has no effect on eye-sight. However, given that we do not know (although we may believe it, and it may be true) that God does exist, it is a far less absurd proposition than any of the above.
Regards,
cXXo