According to this source, a relationship between Esperanto and spiritism has first been mentioned in an article by J. Camille Chaigneau in 1908 in the journal "Gabriel Delanne". Two other books stress the special role of Esperanto: "La Misio de Esperanto" (19.01.40) and "Esperanto kiel Revelacio" (19.01.59) by Francisco Cândido Xavier who said he received their content as a medium from two different spirits, Emmanuel and Francisco Valdomiro Lorenz.
According to this belief, Esperanto has been created to solve a language problem in the (invisible) spirit world.