This claim, frequently encountered on the internet and widely circulated by the religious right, serves as an excellent example of the lengths to which accomodationists will go to challenge the plain meaning of Jefferson's words. No one knows where this claim originated, but it was popularized by religious right author and anti-separationist activist David Barton in the first version of his hour-long videotape "America's Godly Heritage" (a second version omits this claim; see Rob Boston, "Sects, Lies, and Videotape," Church and State, April 1993). Additionally, Barton uses the "one-directional" language, without directly attributing it to Jefferson, in his 1989 book, The Myth of Separation, p. 42. Indeed, the claim was so widely accepted in religious right circles that it was repeated by the head of the Colorado branch of the Christian Coalition before the 1992 Colorado state Republican Party convention ("Sects, Lies, and Videotape," Church and State, April 1993).
Barton's claim is that Jefferson makes the following statement about his "wall" metaphor in his letter to the Danbury Baptists:
That wall is a one directional wall. It keeps the government from running the church but it makes sure that Christian principles will always stay in government."
Needless to say, Barton's claim is pure fantasy. Jefferson made no such statement, either in the Danbury Baptist letter or in any of his other writings. No professional accommodationist scholar gives Barton's claim the slightest credence. Still, the story continues to circulate, and has now become so widely disseminated among religious right activists that it has all but assumed the status of a religious "urban legend."
Barton's "one directional" wall story is only one of the many ways that the religious right attempts to discredit Jefferson's staunch separationism. For a more complete explanation of Jefferson's beliefs, look here.