BibTeX:
@incollection{ShoghiEffendi1938,
  author = {Shoghi Effendi},
  title = {An Instrument, Not a Substitute},
  booktitle = {The World Order of Bahá'u'lláh},
  year = {1938},
  publisher = {Bahá'í Publishing Trust},
}

APA:
Shoghi Effendi. (1938). An Instrument, Not a Substitute. In *The World Order of Bahá'u'lláh*. Bahá'í Publishing Trust.

MLA:
Shoghi Effendi. "An Instrument, Not a Substitute." *The World Order of Bahá'u'lláh*, Bahá'í Publishing Trust, 1938.

Chicago:
Shoghi Effendi. "An Instrument, Not a Substitute." In *The World Order of Bahá'u'lláh*, Bahá'í Publishing Trust, 1938.