"Why do nonbelievers seem to be threatened by the idea of God?",  Rabbi Gellman asks.

I think this question is fueled by projection.  I personally know no atheists who feel threatened by the idea of god.  I do know of some (including myself) who are frustrated and annoyed with peoples' consistent and intrusive attempts to "convince" us of the existence of their god, and of their non-acceptance of us as we are.
I was born in Brooklyn to a mother who was raised in an orthodox Jewish home.  My grandfather was a kosher butcher.  One of his five children maintained orthodoxy throughout his life.  Another was observant, but open-minded, and also embraced Buddhist philosophy and other things that made sense to him.  The other three kids ranged from agnostic to atheist.  I lived in Israel from the ages of 5 to 7 1/2.  There I was taught "God created the world in 6 days and on the 7th, he rested."  At home I had books on dinosaurs, cave art, and prehistory.  At the tender, young age of about 6, I asked my mother which theories were correct.  She explained to me that different people have different beliefs and that I should believe what makes sense to me.   I was relieved when she said that because I did not want to be forced to believe what seemed to me to be a fairy tale.
 
I have been a registered nurse for over 25 years.  Though I don't work in the clinical area at the present time (I work in Quality Improvement), I have always respected others' beliefs and supported their individual spiritual needs, realizing that to some people these beliefs and rituals are extremely important.
Because I have always been aware that I have an intense empathy for people and animals, I consider myself a spiritual atheist (a concept many people have trouble understanding).
I find your theory that something drastic must have happened to cause us to become atheists quite sad.
Although I was raped by 8 men at the age of 16,  I'd already been an atheist for 10 years! 
 
It took me a long time to come to terms with what happened to me. I don't think I fully healed until years later... when while working as a nurse I encountered an 82 year old patient who had been raped.  She was badly beaten and had multiple fractures including her jaw and several ribs. She refused to eat (in her case, drink-as her jaw was wired) and she would not speak...to anyone. I went into her room, closed the door, held her hand, and told her my story. She burst into tears and hugged me, explaining that she was so revolted by the thought of having been forced to perform fellatio on her attackers that she could not imagine using her mouth for anything again...we held each other and cried together... I assured her that it was possible to recover from such an experience. She began to speak, to eat, and to heal.
 
 
I am not a doom and gloom person.   I don't identify with the "existential despair" evident in the statement “The purpose of life is that it ends."  On the contrary, the purpose of my life is to improve the world in any way I can.
 
And, let's just say the rabbi is right and there is a god.  Obviously, he/she/it created some of us with the ability to DOUBT.
 
My mother taught me respect and compassion for people and animals.  She taught me to care about the world.  She taught me to treat others as I would like to be treated. 
I have no problem (other than a twinge of sympathy) for anyone who needs the guidance of a book (any book, not just the bible) to prevent them from hurting others and to keep them on a positive track.
I have just never found I've needed one.
 
And, as for Rabbi Gellman's opening statement "I think I need to understand atheists better", yes, it's apparent he does.
 
Rina Deych