The Letters' file on them is sketchy, but, apparently, they were hard-core saboteurs.
The Judah Initiative is a secret society of Rabbis which originated in Europe and was formed to fight the Thule Society.
In Vitsyebsk, Belarus following the Nazi occupation in World War II, the Thule Society established a camp where they carried out terrible magical experiments on the Jewish people in attempts to reanimate the dead. Commander Eckhart devises the experiments and records the results, which eventually lead to him being able to revive dead Nazis, in a red ledger.
The Judah Initiative in retaliation raise a Golem to destroy the camp, but Eckhart uses magic and escapes, leaving the red ledger behind.
In the years since the war, Rabbi Bass has moved to America bringing the Golem with him. He continued the work of the Judah Initiative and has been trying to find the red ledger. He attempts to tell his children, and later his grandson about the Thule Society and his work, but they think they are made-up stories, part of his coping with the horrors of the war.
At his Bar Mitzvah, Rabbi gives Aaron a book on how to manage a Golem. Unfortunately Aaron uses pages to roll a joint of marijuana.
Torvald, a member of the Thule Society, follows Rabbi Bass, eventually causes him to combust after he locates the ledger. A large box arrives at Aaron Bass's home, and inside is the Golem of Vitsyebsk.
Aaron and his Golem team up with Sam and Dean and find the red ledger, and kill Eckhart and some of the Thule members. Sam and Dean offer to store the Golem at the Men of Letters Bunker but Aaron takes possession of it and vows to continue the work of his grandfather and the Judah Initiative.
The Winchesters contact Aaron Bass after suspecting they are dealing with the Thule Society again. Aaron tells them that he has taken up the Judah Initiative's work of wiping the Thule out with the help of the Golem and together, using the Thule ledger, they have wiped out half of the Thule membership.