The letter by Jefferson was written but it was written some 12 to 18 years after the Bill of Rights was adopted. Reading from several sources and from "Men in Black" by Mark Levin, "Jefferson was not in the United States when the Bill of Rights was adopted and had nothing to do with the 1st Amendment." According to Mr. Levine, "the real source of this strict wall of separation language wasn't Jefferson. It was Hugo Black, a justice appointed by FDR. Prior to being a justice, he was a senator from Alabama. And when he was preparing to run for the Senate in Alabama he joined to Ku Klux Klan, a very powerful force back in the 1920s. So in 1947, he was the justice who wrote an opinion in a case called Everson, involving whether or not public transportation could be used to drop off and pick up Catholic school children. Although the Court ruled that it could, he slipped language into the decision talking about "a strict wall of separation between church and state", that's the slippery slope law today."
I haven't read "Men in Black", but perhaps I ought to.