I was thinking about trying to read the Prose Edda again and remembered this bizarre introduction in which the Norse gods were made into real people who over time were worshipped like gods. How common was this belief throughout Europe at the time of the compiling of the Prose Edda, and what were the motivations for doing this?
Euhemerism was a fairly common rhetorical tool used by Christians to distance the pagan gods from any idea of divinity. In De idolorum vanitate, Cyprian euhemerizes the Greek and Roman pagan gods, and Eusebius does the same to Ba'al in his Chronicle, as do Tertullian and Origen.
Snorri's euhemerization hints that deification of the royal lines helped legitimize them in the eyes of their subjects, but obviously doesn't go into it too much further.
It's a theory known as "euhemerism". It was popular among Scandinavian Christian writers as a way to depict the ancestors of the recently converted former pagans as not evil, but rather misled/misguided. The near east (the gods are said to have come from Troy) at the time was seen as the center of the world, and there were attempts to link Scandinavia to this center.
The Prose Edda was written some time after the Christianization of Iceland, and by then it was no longer a fear that people would convert back to the old beliefs. The introduction serves to provide a backstory, a kind of warning that these myths are not real.
Source: John Lindow, Norse Mythology: A Guide to the Gods, Heroes, Rituals, and Beliefs (page 21-23)
"All that conversion required, according to this theory of natural religion, was for Icelanders to regain sight of God. Unlike the pagans whom Icelanders learned about when they translated and read the lives of the early saints of the Christian church, Nordic pagans were not doomed souls in league with Satan. They were merely sheep who had lost their way"
"After presenting a standard medieval view of the world as consisting of Africa, Europe, and Asia, he says that near the center of the earth, in Tyrkland, lies the city of Troy"
"The idea that gods derive from humans whose actions are reinterpreted and deified by later generations is called 'euhemerism,' after the Greek philosopher Euhemeros"
"Although the medieval Icelandic word aesir etymologically has nothing to do with Asia, the derivation of the aesir from Asia-men completed the euhemeristic process"
The theory of natural religion is, earlier in the book, mentioned as people losing sight of God after the Flood and ascribing their descent from the earth, and the movement of the heavenly bodies to a supreme being.
(page 17)
"In other words, it is possible that the continued transmission of poetry about early kings and battles as historical sources required a continuing knowledge of heroic legend and of myth, not as the object of belief or as something associated with cult but simply as stories that people interested in the history of their own culture had to know"