This is the first time I've seen Freemasonry described as an "organization of free thinkers". Wikipedia says "most Grand Lodges [of Freemasonry] require the candidate to declare a belief in a Supreme Being". It also says that "freethinking is most closely linked with . . . atheism [and] agnosticism."
Certainly many freemasons in Franklin's time were deists who did not conform to the religious orthodoxies of the era. I don't know much about the current state of freemasonry.
I agree with cpgatbyu. It's just not correct. While I might describe the freemasons in my acquaintance as "free thinkers," I would never describe Freemasonry as an "organization of free thinkers" (nor would they call it that themselves I'm pretty certain!).
For the purposes of the clue it would be better described as a community-oriented fraternity, or you could just keep it simple with "He joins this fraternity."
When Franklin was a young man deists were the free-thinking atheists of their day. A deist believed in a vague amorphous supreme being, an "absent watchmaker" who may have set the universe in motion but obviously no longer was present or intervening in human affairs, primarily because this was pre-Darwin and pre-Hawking and there had not been theories proposed in their world for the origins of life and the universe that did not involve a god. They all thought Christianity and the Bible were bunk, that "god" if there was one was not watching us, and were generally strongly opposed to organized religion and the things it led to (decades-long wars in Europe, the Inquisition in Spain, the reign of "Bloody Mary," etc).
I don't know enough about Freemasons of the time to say if they were primarily deistic but if Franklin was a member they may well have been and it's entirely likely that they, just like all people religions and things, have changed a lot since the 18th century.
For the purposes of the clue it would be better described as a community-oriented fraternity, or you could just keep it simple with "He joins this fraternity."
I don't know enough about Freemasons of the time to say if they were primarily deistic but if Franklin was a member they may well have been and it's entirely likely that they, just like all people religions and things, have changed a lot since the 18th century.