Maarten J. Vermaseren, Cybele and Attis: the myth and the cult[permanent dead link], translated by A. M. H. Lemmers, London: Thames and Hudson, 1977, p.96: "Furthermore Cybele was to be served by only oriental priests; Roman citizens were not allowed to serve until the times of Claudius."
Firmicus Maternus, Julius; Forbes, Clarence A. (1970). Firmicus Maternus : the error of the pagan religions. New York: Newman Press. p. 81. ISBN0-8091-0039-8. OCLC88488.