Three, Seven, and Thirteen.
Strange Persistence in Nature of These Mystic Numbers, Each of Which Has Ever Been Regarded as Deeply Significant by the Various Races at Different Periods of History, and Especially in Religious Observance.
Superstition of some sort or other has been attached to certain numbers from time immemorial, but the numbers three, seven, and thirteen have been particularly favored, and three and seven have figured very prominently in mythology, scriptural history, and elsewhere. Three is called the perfect number, seven is regarded as lucky, and thirteen as unlucky.
It was Pythagoras who termed three the “perfect number,” because it expressed “the beginning, the middle, and the end,” signifying a perfect whole.
On this account he made it a symbol of the Deity, and the “Holy Trinity” is now, and doubtless will be always, the most potent symbol of Christianity. The world was supposed to be under the rule of three gods: Jupiter (heaven), Neptune (sea), and Pluto (Hades). Jove is represented carrying three-forked lightning. Neptune carries a trident, and Pluto is accompanied by a three-headed dog.