In his writings on mythology, such as "Jocasta's Crime" and "The Hero", Lord Raglan discusses the characteristics of a typical mythological hero. We can list these, with a few additions and amendments:-
*The hero is the firstborn child (often the only child) of his mother, who may be of royal descent, or a priestess
*His father is of royal descent, often closely related to the mother, but:-
*It is believed that his true father is a god
*His birth takes place in unusual circumstances, and is attended by various signs and omens
*Soon after his birth, powerful people attempt to kill him, and he has to be spirited away to a distant place
*Little is known about his childhood and youth, though there may be the occasional story illustrating his prodigious talents
*Embarking on his career, he undergoes some ordeal or religious initiation, or:-
*Alternatively, he may kill a monster, or the existing king
*He then marries a princess, who may be the daughter of his predecessor, and becomes the king
*He may be a warrior-king, but he also prescribes laws and religious observances, and rights social injustices
*Eventually he loses the favour of the gods and/or his subjects, and is driven out, or:-
*He is betrayed to his enemies by someone close to him
*He suffers an ignominious death, often on top of a hill
*He is not succeeded by his son (often he has no children)
*He has no tomb, but:-
*There are several holy sites devoted to his cult
Lord Raglan then runs through the stories of various heroes to see how well they tally with these criteria, mostly from the classical corpus (i.e. Graeco-Roman): Oedipus, Theseus, Romulus etc, but then ranging further afield; through Moses, Siegfried, King Arthur, Robin Hood and Llew Llaw Gyffes to Nyikang (Sudan) and Watu Gunung (Java); noting the similarities of the stories.
Why should there be such extensive similarities? Raglan, drawing upon the pioneering work of Sir James Fraser ("The Golden Bough") wonders whether the myths have spread outwards from a central source (perhaps Egypt?) and are based upon an ancient religious ritual: the Sacred King who is sacrificed every year to ensure the continuity of the seasons and the fertility of the crops. Robert Graves ("The Greek Myths") wonders if the myths are a distant memory of events which actually happened, in the time of the Indo-European migration into Europe in the early Bronze Age. Neither theory explains how similar stories are told in very distant cultures. Perhaps the myths embody ideas deep within the human psyche? Freud suggests this in "Totem and Taboo"
As an alternative, we may ask how reliable is our understanding of the myths and legends of distant cultures. These were mostly collected from peasant oral traditions by 19th century western European researchers, and translated and retold for western European audiences. Both the collectors and their readers would have been steeped in the classical traditions, and would always be inclined to interpret the new stories in that light: they would know in advance what ought to happen to a mythological hero! For the same reason, genuinely historical heroes tend to get suitable legends attached to them, especially concerning their frequently obscure childhood years. Such pseudo-history may become attached to, for instance, Alexander the Great or William Wallace.
Raglan carefully refrains from giving a rating to Jesus considered as a mythical hero, but maybe the same considerations apply. The notion that God could come down to earth and father a child upon a human woman would be anathema to orthodox Jews, but very familiar to anyone brought up on the Classical myths. We are likely to have a convergence of stories, from history through pseudo-history, legend and mythology to pure fiction: from William Wallace through Robin Hood and King Arthur to Hiawatha.
The theories of Raglan and Graves are generally discounted by modern scholars, but have been very useful to novelists: for instance, the ritual sacrifice of the Sacred King as being an actual historical event features prominently in Mary Renault's "The King Must Die"