The only two letters with anything to do with a "separation". The first is a letter from a Danbury minister to Jefferson, then Jefferson's reply. Basically the minister is concerned the new government is powerful enough to start making laws effecting religion. Jefferson says there is a separation, and the government cannot legislate on religious issues. Tell it to Frank and check his posts. He believes the opposite.
Let me also point out that this thread was 2 years old when Frank called it up to argue for atheism in public life.