1560s, "pertaining to or consisting of a rite or rites," from French ritual or directly from Latin ritualis "relating to (religious) rites," from ritus "religious observance or ceremony, custom, usage," (see rite). By 1630s as "done as or in the manner of a rite" (as in ritual murder, attested by 1896). Related: Ritually.
1640s, "prescribed manner of performing religious worship," from ritual (adj.). From 1650s as "book containing the rites or ordinances of a church," also "the external forms of religious or other devotional exercises," often in that sense somewhat pejorative (mere ritual, forgetful of meaning).