The Courtier's Reply is an "argument" one often hears from believers. Calling it an "argument" is actually too generous--conversational gambit would be more accurate.
The Courtier's Reply is a logical fallacy in which someone seeks to refute another person's point by claiming that the person can't be right because he is not an expert in the field. (Notice that it is also a form of ad hominem, because it is an "argument" based on the person and doesn't actually reflect the subject.)
It is called "the courtier's reply" because of its alleged origin in the story of the Emperor's New Clothes. Supposedly, one of the king's courtiers heard the little boy saying that the king was naked and replied that he only thought that because he was not an expert on regal clothing.
A good first response is:
"I don't have to be an expert on royal clothing to know when the king is naked."
"I don't have to be an expert on bullshit to know it when I see it."
Depending on the context, a more erudite and pointed reply might be:
"Theology is just the study of what other humans have had to say about their invisible friends. It is meaningful only if the invisible friends exist. Like almost all the arguments made by theologians, yours fails because it is circular--it assumes your conclusion is already proven."