Psychology and God: The Genetic Fallacy

I was recently in a conversation with an atheist on FB who I've known for probably 35 years.  He's an intelligent guy and is pretty feisty (as am I.  Feisty that is, me being intelligent could be questionable).  It got a little heated but he did throw a challenge my way that after considering it I thought I'd do a little more homework.  His interest lies in the area of psychology and the basic claim is that God is something people have made up to fulfill emotional needs.  I'm boiling the claim down but I think that captures what he was trying to say.

I've studied the psychological arguments atheists make but my real interests are in the area of the cosomological, teleological, or moral arguments in favor of theism.  So I thought I'd give some treatment to my thoughts about the idea that God is made up in our minds to help us cope with our world.

Ludwig Feuerbach was one of the first to suggest that God was the product of the human mind.  He certainly hasn't been the only prominent figure to assert that.  Nietzsche, Marx, and  Freud all followed along those lines as have others after them.  One of the problems with this argument is the genetic fallacy.

The genetic fallacy is when an idea is rejected because of where it came from.  This is a disapproval of the source of the idea rather than the content of the idea.  So how does this relate to people wishing God exists?  Let me give a couple examples and maybe it will be more clear.

Pythagoras was a Greek thinker who was also a religious mystic who worshipped numbers and geometry.  However he also is the father of the Pythagorean theorem.  Suppose someone were to reject the Pythagorean theorem because Pythagoras had some wacky mystic ideas.  That would be fallacious because it doesn't follow that because the source, Pythagoras, had mystical ideas about numbers that his theorem is wrong. 

Here's another example from CS Lewis in God In The Dock:

Suppose I think, after doing my accounts, that I have a large balance at the bank. And suppose you want to find out whether this belief of mine is 'wishful thinking.' You can never come to any conclusion by examining my psychological condition. Your only chance of finding out is to sit down and work through the sum yourself....If you find my arithmetic wrong, then it may be relevant to explain psychologically how I came to be so bad at arithmetic...but only after you have yourself done the sum and discovered me to be wrong on purely mathematical grounds....In other words, you must show that a man is wrong before you start explaining why he is wrong. C.S. Lewis

Whether someone wishes God exists or it gives them comfort that God exists has nothing to do with the actual truth of whether God exists or not.  People have all kinds of crazy ideas about things that are actually true.  Psychological motivations have nothing what so ever to do with whether something is true or not.  The truth has to come from somewhere else outside of psychology.  Saying that psychological motivations prove something is true or not is committing the genetic fallacy.