As a part of his missionary work, he learned Persian and studied Islam, particularly Shi'a Islam, Persia and Persian culture. During his missionary work in Persia, he also encountered followers of the Baháʼí Faith, a large religious minority there. Much of his ministerial work involved developing Christian apologetic responses to these religions.
With respect to the Baháʼí Faith, his Baha'ism, Its Origin, History and Teachings,[2] published in 1931, was the first of two books, and several apologetic and polemical articles on the subject.[3] Along with Earl E. Elder he translated Bahá'u'lláh's Kitáb-i-Aqdas, one of the central books of the religion; this translation was published by the Royal Asiatic Society in 1961. In 1974, a new follow-up, The Baha'i Faith: Its History and Teachings[4] added additional material not available previously, made available to the author by a known opponent of the Bahá'í Faith.
After 1962, he retired with his wife to Mount Airy, Pennsylvania, where he lived until she died, and then went to live in a retirement home until his own death in Philadelphia, Pennsylvania in 1993.
Works
'Translation of Al-Babu'l-Hadi' Ashar by Al-Hilli, a treatise on the theological doctrine of Shi'ite Muslims'; London, Royal Asiatic Society, 1928.
^Elwell-Sutton, L.P. (1976). "Review of "The Baha'i Faith" Its History and Teaching by William McElwee Miller". Journal of the Royal Asiatic Society of Great Britain and Ireland. 108 (2): 157–158. doi:10.1017/S0035869X00133416. JSTOR25203713. S2CID162531277.