The Mendai are the other people the period called Sabians, and Adams’s own name for them, Christians of St John, is the disambiguation the archive uses against the Harranians.
In Hannah Adams
In Adams’s own words, from the 1817 Dictionary. Each passage is quoted verbatim under its own entry.
MENDAI, or MEnDzANS, otherwise called Christians of St. John, or Hemero Baptists, which see. ** These ambiguous christians (says Mosheim) dwell in Persia and Arabia, and principally at Bassora, and their religion consists in bodily washings, performed frequently and with great solemnity."
Hannah Adams, A Dictionary of All Religions and Religious Denominations, 4th ed. (Boston: Cummings and Hilliard, 1817), entry “Mendai” (p. 172).