What is the name and who is the author of a sci-fi story written about the the writing of the bible?

It starts with two brothers (I think Aaron & Jacob) and one says to the other that in the beginning, about xxx billion years ago, the universe was created, and the brother chimes in that normal people couldn’t understand that length of time, so xxx million years is suggested. They argue some more and eventually the last line (ish) of the story works out to be something like “On the first day…”