[ClearCase_guy]

I can't make a definitive statement about the Federalist Papers, but one standard mention that people point to is a letter Thomas Jefferson wrote in 1802 that suggested that "there ought to be" a wall.

The concept is entirely unofficial.

[Don W]

Mr. Jefferson referred to “a wall of separation” in a letter to a Baptist leader, but it was to protect the churches FROM the state, not the other way around!

[Joe 6-pack]

Just to add to what CCG said - the context of Jefferson’s letter to the Danbury Baptist Church regarding separation of church and state was to allay the fears of clergy regarding the prospect of government overreach into religious life. Perhaps the greatest irony is that Jefferson’s closing of the letter would most likely aggrieve the modern atheist/nihilist as violating the very wall they continuously cite out of context.