As a triangular number and the product of the sacred seven and thirteen, ninety-one has a form that reinforces its arithmetical interest, and its reduction to one ties it to unity. Beyond the gematria of amen and the union of the Names, the number carries little narrative lore. Its honest and genuinely elegant core is that single equation, amen equals YHWH plus Adonai equals ninety-one, which gathers the affirmation of prayer and the two names of God into one figure.
The whole apparatus of digit-reduction numerology is a twentieth-century invention rather than an inheritance from antiquity. Mrs. L. Dow Balliett published the first systematic version, The Philosophy of Numbers, in 1908, fixing the letter-to-number chart still used today and ruling that eleven and twenty-two must stand as master numbers rather than reduce further; her colleague, the physician Julia Seton, is credited with coining the word numerology itself around 1912.
A separate and older-claimed system, generally called Chaldean numerology, assigns letters to digits irregularly rather than in Balliett's repeating one-to-nine sequence, and deliberately omits nine from the chart as too sacred to attach to an ordinary letter; its popular billing as a genuinely ancient Babylonian system rests on assertion rather than on any textual evidence that has been traced back that far.