Time and again, I found this letter described as being written to Paine. I found it on quasi-religious, patriotic sites accompanied by self-satisfied commentary that Paine was buried in a field because no American cemetery would take his blasphemous body. I found it on sites declaring that they were supplying primary source documents. I found it on Wikipedia's article on Benjamin Franklin. I found it on an atheist forum.Franklin's reasoning, that religion is good for the mass of men to keep them from wild misbehavior, has a long history. Many of the Greek philosophers assiduously attended the civic religious celebrations of gods they no longer fully believed in, because they felt the peace and order of the city was an enormous moral good. Confuscianism has a similar focus on civil harmony.
Terri found it harder to get to the truth than one might think.
BTW, I would guess that she composed her title after reading through all the 18th C material. I find myself doing the same thing, moving toward the style of speech or writing of who I am currently reading. Great fun when it's Wodehouse or Doyle.