As long ago as the 4th century B.C.E., a student of religion and myth named Euhemerus theorized that the gods and their stories had their origins in actual historical events. His name has given us the term euhemerism [yū-hē’mə-rĭz’əm]:
interpretation of myths as traditional accounts of historical persons and events –OED
Much later, an Icelandic student of the Norse myths, Snorri Sturluson (1179-1241), suggested that the gods began as human war leaders. He speculated that cults grew up around the burial sites of kings and renowned warriors. The living visited the sites called upon the departed for help in battle. Eventually the dead heroes were remembered as supernatural beings with the power to bestow victory in war.
Related words are:
euhemerist: noun, one who follows the method of Euhemerus
euhemeristic: adjective, ) of persons: Inclined to euhemerism; (b) of things: Of the nature of or resembling euhemerism
euhemerize: verb, To subject to euhemeristic interpretation; To follow the method of Euhemerus.