The stories of the goddess were originally the closely-guarded secret of the temple custodians. These stories were however passed to the common people disguised as entertainment. When they became part of the common folklore they were changed as people tried to make the goddess represented in the tales behave more like an ordinary person... for example if a story said that the goddess gave birth to such-and-such a god or nymph, then the later stories would provide her with a husband or a sexual encounter to explain this. In this way, the myths soon became very tangled, and when we unravel them we find that the goddess has married herself under different names; has given birth to herself several times, and even made war on herself under different names.
In later centuries, there was another reason to keep the goddess stories secret. From the Greek times forward, the goddess religion was suppressed and forbidden, and to declare any expertise on the subject was to risk death. This was the case in the middle ages in Britain, where the tales were still told, but in forms which only the knowing would recognise, and with plentiful references to Christian piety in case the authorities got too close. Chaucer's Canterbury Tales (particularly The Miller's Tale) and the Robin Hood stories are other examples, but the most revealing may be that of King Arthur.
The story of Arthur contains a full suite of the Celtic version of the goddess myths. The principal story, that of the two kings representing the old and the new years, who contend for the affections of the goddess, is given in the relationship of Arthur; Lancelot, and Guinevere. Guinevere (whose name means Lady in White) is the goddess. Arthur is the god of the old year, and when he goes away he is replaced by the god of the new year, Lancelot. This implies infidelity on Guinevere's part and introduces a tension in our understanding - if she were an ordinary mortal, that is, and if this were an ordinary story about ordinary people.
James Archer's Death of Arthur
Guinevere is not the only face of the goddess in this story. Arthur is given his kingship by the Lady of the Lake (Nimué), who we may identify as being the goddess by her magical powers and her association with water. When Arthur dies he is taken by three ladies (representing the Triple Goddess) in a boat (the tradition) to Avalon (the goddess' apple isle, the traditional paradise reserved for heroes). A Mediterranean version of Arthur would have been taken instead to the Pleiades.
Merlin, the model for future wizards, is an echo of the Celtic high druid or ollave, a role first played by the high poets of the Mediterranean goddess-temples.
As has been said above, the keepers of the original goddess stories were under an obligation to keep the stories both secret and unchanged. This tradition was apparently still alive at the time of Sir Thomas Malory. In his Death of King Arthur Malory rebukes other writers for having recorded the story. He was referring to French authors who picked up the story from Welsh poets, who in turn had got it from the older Celtic bards. Malory also takes care to fill his book with heapings of Christian camouflage. When Arthur dies, Guinevere becomes a nun, and Lancelot a monk and so on.