...The King of Rock 'n' Roll
Elvis Presley was and still is so popular that there is an entire line of work dedicated to impersonating him. Some impersonators are really good; some are really bad. The challenge offered by the atheist appeals above to the Christian's (and broadly, society's) two understandings that many who have claimed to be or were understood to be God have been shown to be false. Let's start with the claims then move to the understandings.
I have chosen the example of Elvis to show just how illogical it is to reject the existence of the real thing just because impersonators exist. We all recognize that Elvis Presley did exist, and we recognize that the impersonators are just that: impersonators. The fact that impersonators exist does not logically force us to conclude that Elvis did not exist. But, let us forget that this is so obvious for a moment and look to see how it can be so obvious. What is it that we know about the real Elvis Presley that is different about the impersonators that allows us to accept one and reject the other? While fans of Elvis are going over their list of impersonator pet-peeves in their minds, I'll just bring up just one. My understanding, as well as most everyone else's, is that Elvis has passed away, while the impersonators are very much alive. This alone allows us to remove all impersonators from the possibility of being the real Elvis Presley, and the reason why they are called "impersonators." (*see note below)
However, many other attributes of the real Elvis Presley can also be brought to strengthen our acceptance of one but not the other. The concept of Elvis Presley is what we examined versus what we see.
Creating God In The Image of Man
This is exactly what we do when we reject numerous gods yet accept one. Over history cultures have had numerous concepts of god. Everything from super-human (such as Thor) to super-natural (such as Yahweh). Some concepts of god mirror man so closely as to even include man's fallen nature, while others are so different from man that they are incapable of personal relationships. Because there are so many different ideas of who "God" is, there is very little logical connection between them that would necessitate rejecting one god if another is already rejected. The only way that it would logically make sense to reject an entire category is if it can be demonstrated that a common attribute of the concepts of God is impossible or incoherent. In that case, any god that contains that attribute could be taken off the table, while others who do not possess the impossible attribute are still viable.
Christians understand that their God is the only God left standing after filtering through the impossible and incoherent attributes of other gods. They do not believe that the other concepts of God are accurate, thus they reject them. The rejection of the other gods is rational, while the belief that the Christian God exists is also rational. Part of studying theology is coming to an understanding of God's attributes. When we properly understand the attributes of God, we see that there is no incoherence between them and reality or among the individual attributes. For if it can be shown that there is incoherence in a god, it has also been shown that that god does not exist in reality. As a defender of the Christian worldview, I often answer claims of incoherence among God's attributes, showing how two (or more) attributes do not conflict with or contradict one another. It is often brought out that other concepts of god do possess the conflict or contradiction, so they are not viable, while the Christian God is.
Conclusion
It does not logically follow that because we reject one entity, that goes by the descriptor "god," that we must reject all others. The only logical test for such rejection is to examine the concepts and remove them as we find that they do not reflect reality. The atheist's request of the Christian to reject God's existence on the grounds that they have already rejected Thor's existence is as logical as asking anyone to reject that Elvis Presley existed because impersonators of him exist.
*Did I really just say that we can know that who we see is an impersonator because we know Elvis is DEAD? If I accept this, how in the world can I accept that Jesus resurrected based on people seeing him? Simply this: close friends and family are not fooled by impersonators. The disciples and Jesus' brother (James) went to their deaths because they believed that who they saw was actually Jesus after he died, not someone claiming to be him. No member of the Presley family or his close friends is going to get themselves killed because they believe they saw him at a party or casino. They know Elvis' authenticating attributes, just as the disciple Thomas knew Jesus' authenticating attributes of having nail-scared hands and a pierced side.
Check out these great posts from others on this same topic:
Atheists Disproving The Wrong God
Dissecting 'One God Less' Meme
On The Statement That "We Are All Atheists"