The Celts were fundamentally different from many of the other pre-Iron age peoples that invaded and then settled in what is now called Northern Europe.
That they were polytheists comes as no surprise, most pre-Iron age peoples were. What made them unusual and different from the many warlike races that roamed Europe at the time was their belief that only a narrow veil separated the living from the dead. Your loved ones, the dead, were only an arms breadth away. They were always close to you, watching what you were doing and waiting for you to join them in the otherworld, that lay so close to our own.
This “Otherworld” was neither a heaven nor a hell, rather another plane that you moved to after death where everything was pretty much the same as the one that you had lived in.
There was only a short space between the worlds of the living and the dead. A space that could be broached by a strong presence or a loving thought, a space that grew ever narrower at certain times of the year.
They imagined that the veil that separated the lands of the living and the dead was subject to the changing of the seasons, where at certain points in the calendar, Samhain and Beltane, (The End of winter, the beginning of the rebirth and the reverse) would allow that veil to be easily parted. This gave rise to Halloween, the time when the veil was so thin that creatures and ghosts could broach the veil and enter the world of the living.
Halloween is simply a Scot’s language bastardisation of the Christian festival “all Hallows Eve” the day before “all saints day” but the Celts believed, before Christianity took their thoughts away in a whirlwind of saints and martyrs, it was their own loved ones and their own misdeeds and failures that had come to haunt them.
Additionally, they believed in the “others” the fae, faerie, Sidih, Shee, Fairy, fee that go by so many names that I cannot recall them all.
They believed that these creatures of the “other” were neither human nor their dead come to haunt to them but rather were of another type of human. A being of magic that existed not on the same plane as humanity but rather was removed from the normal much as their dead were.
I have used this Celtic philosophy in many of my tales and know that it is fanciful, which is why I call the majority of my tales’ fantasies but there is a certain beauty in the thought and who can say what is true and what is simply imaginings.
I like the idea that your dead are still close to you and can still feel you even after they have passed the veil, I enjoy the thought that creatures of dread can part that veil and somehow become real in this natural world and so I tell my Tales. The latest is “Trow” which I can only say fits the moniker of “Tales of the Unexpected”