Christian symbolism connected the prickly leaves with Jesus' crown of thorns and the berries with the drops of blood shed for humanity's salvation, as is related, for example, in the Christmas carol, 'The Holly and the Ivy'. Yet even here the reference to these two plants refers to a pre-Christian celebration, where a boy would be dressed in a suit of holly leaves and a girl similarly in ivy, to parade around the village, bringing Nature through the darkest part of the year to re-emerge for another year's fertility.
Holly was also brought into the house variously to protect the home from malevolent faeries or to allow faeries to shelter in the home without friction between them and the human occupants. Whichever of prickly-leaved or smooth-leaved holly was brought into the house first dictated whether the husband or wife respectively were to rule the household for the coming year.
In Celtic mythology the Holly King was said to rule over the half of the year from the summer to the winter solstice, at which time the Oak King defeated the Holly King to rule for the time until the summer solstice again.
However the folklore of the holly is not solely connected with Yuletide festivities. Like several other native trees it was felt to have protective properties, and there were taboos against cutting down a whole tree. Hollies were frequently left uncut in hedges when these were trimmed. A more arcane reason for this was to obstruct witches who were known to run along the tops of hedges
Holly trees were traditionally known for protection from lightning strikes, to which end they were planted near a house. In European mythology, holly was associated with thunder gods such as Thor and Taranis. We now know that the spines on the distinctively-shaped holly leaves can act as miniature lightning conductors, thereby protecting the tree and other nearby objects. Modern science occasionally catches up with an explanation for what may previously have been dismissed as superstitious lore!