BibTeX:
@incollection{ShoghiEffendi1941,
  author = {Shoghi Effendi},
  title = {The Promised Day: The Decline of Established Religion},
  booktitle = {The Promised Day Is Come},
  year = {1941},
  publisher = {Bahá'í Publishing Trust},
}

APA:
Shoghi Effendi. (1941). The Promised Day: The Decline of Established Religion. In *The Promised Day Is Come*. Bahá'í Publishing Trust.

MLA:
Shoghi Effendi. "The Promised Day: The Decline of Established Religion." *The Promised Day Is Come*, Bahá'í Publishing Trust, 1941.

Chicago:
Shoghi Effendi. "The Promised Day: The Decline of Established Religion." In *The Promised Day Is Come*, Bahá'í Publishing Trust, 1941.