Before an epiphany was a lightbulb moment, it was a god showing up.
The Greek epiphaneia meant a manifestation or striking appearance. It also named festivals held to mark a god's appearance at a particular place. The word is built from epi-, "upon," and phainein, "to show." The related verb could mean "to come suddenly into view." Suddenness was part of the word from the start.
Christianity took it over. In the New Testament, epiphaneia refers to Christ's advent or manifestation. The feast of the Epiphany, on January 6, celebrates the moments his divinity was revealed. Britannica lists three: the visit of the Magi, which stands for his manifestation to the Gentiles, his baptism in the Jordan, and his first miracle at Cana. The feast is also called Theophany, "manifestation of a god," or Three Kings' Day.
The Latin form was already known in Old English. The word as we have it settled in by the early 14th century, shaped by Old French epiphanie. For centuries it meant the feast and little else. By the 1660s it could describe the appearance of other divine beings. By 1840 the writer De Quincey was using it for any manifestation or revelation.
The modern meaning came from James Joyce. In Stephen Hero, written in 1904 to 1906, his young hero defines an epiphany as "a sudden spiritual manifestation, whether in the vulgarity of speech or of gesture." Joyce collected more than seventy of these moments from ordinary Dublin life, and about forty survive. He used the device in five stories in Dubliners (1914). Critics then borrowed his term for the flashes of insight in modernist fiction by Woolf, Proust and Mansfield.
The feast also left a quieter trace in English names. Girls born on the feast day were sometimes named for it, from Old French Tiphanie, a form of the Latin Theophania. That name survives today as Tiffany.